As always, just my opinion, and I'm not interested in making others' opinions closer to mine:
From my point of view, it doesn't matter how old a thread is, or if its creator is reading or not.
I see two reasons for these threads.
One has a secondary level of relevance, and another one is the big reason IMO.
The secondary one is what us, present day forum members, can learn and teach from/to ourselves while we discuss.
The main one is what a lot more people, beyond the few of us alive yet, will read in the future.
For both groups, and about OP's question why:
1) Metering and placement differ: apart from metering differently, some people place shadows' darkest details in zone 3 while others prefer zone 4: this means people actually talk about EI400 and EI200 but they may be talking about the same exposure exactly.
2) One of the reasons for giving a negative more light than box speed is, with a stop more light than box speed, grain is less visible. Anyway, Tri-X is a film for grain presence, compared to other films.
3) ISO400 films behave differently. At EI400, for instance, both Tri-X and HP5+ can be used, but IMO even though both show different, usable tonalities at box speed, Tri-X seems to benefit more than HP5+ with a stop more light. This makes me use HP5+ at EI800 commonly, without the slightly high contrast that implies for Tri-X: both films are fine for overcast at EI800, but HP5+ has a lower native contrast that's better not only for pushing, but also for mixing overcast/sunny scenes in the same roll without blocked highlights/shadows when I go to EI800. That's very important for street, and has no relevance for tripod work.
4) It depends on film and developer: some developers (e.g. metol ones) make films look better with a stop more light than box speed. But that stop more light makes no sense, with the same film, when a different developer is used, say D-76 or ID-11.
5) Most considerations are important for 35mm photography, but they're different or nearly irrelevant for larger formats.
6) Exposure/development under soft light requires less precision than under direct sunlight: as for soft light scenes we use just a part of our negative's latitude, we can expose at 100, 200, 400, 800 and 1600, and we'll be able to find a development time/agitation scheme that makes a good negative for wet printing. But under direct sunlight, we use all our film's latitude, so things really benefit from a more precise behaviour. As I've written here many times, students get confused because they read half truths: things that are true for overcast only, or for direct sunlight only, and some posts/books don't specify that.
7) My recommendation for students is they should meter at box speed always, and in soft light always: never under direct sunlight. Then they should develop in a way that makes it possible to print soft overcast negatives with multigrade filters close to number 3, and more contrasty sunny negatives with filters close to number 1. This implies development times that are shorter/more diluted/less agitated than those recommended by manufacturers, which are in general better for medium contrast scenes only.
8) Scanning doesn't allow us to see the real/optimal use of photographic materials, because real film's tone disappears inside the hardware/software characteristics and protocols as soon as the all-new tone digital photograph is made by the scanner.
9) For sheets, because of the possibility of their individual development, we can use the zone system: meter the shadows, meter the highlights, check the difference, and decide placement and development. That's easy, I mean, clear and with few doubts. But when we mix overcast and sun scenes in a roll, it's a different story. Not only because of those two very different types of scene contrast, but also because under direct sunlight shadows vary drastically: depending on bounced light and clouds, and on the other hand buildings, trees, etc., those shadows range from 3 to 9 stops less light than direct sunlight, so in case of mixed scenes, what we must care about is not burning highlights (gentle development), but we can't really expose for the shadows: darker shadows will be darker, while scenes with cleaner shadows will show that too. In other words, individual negatives allow us to use the zone system, while rolls for mixed scenes remind us of slide film exposure and development: those care for highlights, letting different types of shadows fall where they were in the real scene.
10) Another BIG fact for confusion is all data and results come from two different types of testing: some of us test for the "speed" of medium grays, and some of us for the "speed" of Zone I: both imply considerable differences.
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