IME the writing in the film rebate isn't particularly reliable as an indicator - at least standing on its own. It tends to vary a bit even when your exposure and development are consistent.
The rebate writing is exposed at the time of film manufacture. If you (or the manufacturer) expose film at one time, and than wait a long time to develop it, the latent image will degrade over time. Some films suffer more from this than others, and storage conditions play a role as well.
I can recall at least one occasion in the past few years when Ilford/Harman were having some problems with the machine that exposed the rebate images. That sort of inconsistency is rare, but not impossible.
The edge writing is a better indicator if you are comparing two films from the same batch exposed and developed by you at times close to each other. Changes in the density might be indications of inconsistency in your procedures.
By the way, I'm assuming you mean HP5 Plus, because HP5 film would be very old.