Not sure about the Internal count, as I do all this on my phone and it makes everything tricky, but I think you are right.
So, where does the difference in cargo come? With the 9 I can fit 5A Shields, throw on some boosters if I want, and get 300+ shields. More then enough to submit, boost, and high(low wake) out. With the T10 I can't use 5 shields because of the 1200t I am guessing. Therefore I end up with less cargo capacity.
Yep I just tried it, with the T10 you need at least class 6 shields, the class 5 won't work with the extra mass. So if you run shieldless then the T10 can carry the same cargo as the T9, but if you run with shields then it carries 32T less.
But then with the T10 and those two C5 MIL slots you can just hull tank interdictions and forgo the shields completely, hah!