As we wander in the conversational desert hundreds of kilometers from my original posting question.......
Paul - what you stated last is not realy "untrue"! ....sorry

...
So coming back to your question :
........so where is the difference oft this two films?????
The first one is from Harman/Ilford like I stated in your other thread - pls. remember!
Others have same sugestion including yourself - obviously!
It has to be a Kentmere derivate film!
So one might " speculate" Photo warehouse made contracts with Ilford!
Then we should ask: is it the same contract with Ilford - Photo warehouse made again?
THEN IT SHOULD BE DELTA400 - if it wouuld be such easy

! From my point it can't be!
Generally (Ilford would never give their "best horse" away for cheap pricing )- they are not silly!
The fact that it possible was done in the past has had other reasons (overproduction)!
Is that the possible reason again : overproduced Delta400 ? From my point : never - because Ilford
would have the possibility to smaler their expensive pricing with Delta then!
From the same reason the could make exact this deal with photo warehouse - they hold expensive
pricing with Delta and let photo warehouse some films for cheap offerings. (The all times mechanism with brand - named films)....hmm

?
I can not belive that Ifords bestseller have problems at this moment!
But who cares about what one can belive

? You stated the characteristics are not like any
Tgrain you remember - is your workflow suddenly such bad that you have "destroyed" the films
characteristics via development??? That we all also "can not belive" - right ?

!
BTW : is there a mark onto the package or the cassetes : Made in Europe ?
But I won't think - it isn't Fujis "Presto" (discontinued 2013).....hmm!
KodakTmax400 isn't avaible for brand manufatured films (be sure about 100%) - Kodak could need any money today - more than ever before - but to the same side : They are no ideots
If it is a real Tgrain Film : It must be Delta 400 - but it isn't a Tgrain - right?
You may have a last prove Paul : Shot one Tmax400 and one Delta400 and develope in exact same direction - with your films you still have shot from photo warehouse - will it more grainy indeed
the film has a massive defect (expiration more than 5- 8 years ago for example + bad storage) or it isn't a Tgrain film !
But from my point AS stated before : Then it could be ans film

! I have no idea what it could be then!
with regards
PS : I would change to TMY2 - or I would buy a cheapest film = normal photography : there is business AS usual / photography were you need realy good results = best films!
But that is your personal decision! I often shot bw "in addition" when I am shooting in color!
Therefore I personally don't need bw films from highest price = a normal APX100 will do this job!
But if it goes about bw shooting exclusevly I also need a best film!