I get an EI of 200-250 for Tmax 400 in D76 1+1 as well but why does that matter to you if you are satisfied with negs and results? if EI250 gives you all you need and has sufficient shadow detail, go for it.
This thread documents how I calibrate my sensitometer, so it's important to know the film speed so I can work backwards to deduce the exposure. But that's theory. In practice you are right, it doesn't matter - because when I use EI 250 on my meter... my shadows have the detail I want.