torx sizes

for my sanity, i have a quick q i couldnt answer via searches

have the MB and MBP always used different torx heads relative to HDD replacement?
Or did the change occur with the shift to unibody?

I know my white macbook used T-8 Heads because I had to buy one explicitly for the MB.
Now, going to swap my new MBP drive, and it needs not the T-8, but a T-6 head.

GRRR....I cant see why you'd change head sizes between models or versions, for a relatively common need (HDD) whose specs havent really changed that much (not to the extent of needing a different head on the screws....)

 

Hmmm. Odd that the sizes differ between mb and mbp