Wilt, the glass of the front element of the 24-70 MkII has a diameter of about 65 mm. I just measured it!
I can't tell you why Canon made the flipping filter diameter 82 mm... It makes everything so much more expensive...
..And since you are an enquiring mind, I'll give you for free the front element diameter of the EF 70-200 MkII, approx. 75 mm and it takes 77 mm filter
Conversely, the 18-200 measures 67-68 mm and it takes 72 mm filters while
the old Vivitar Series 1 70-210 has a 55 mm front element and it takes 62 mm filters.
If you are any the wiser, let me know... I am not!... 
EDIT: Maybe there is a trend, the FOV of the 18-200 is actually less than the FOV of the 24-70 on a FF. So, maybe they need the extra real estate so that you can't see the edges of the filter.

Too bad Oly couldn't figure out how to do it and had to go M4/3 with an EFV.


