And really? You'd buy a 7.85” but not a 7”? Why? Because apple says 7” is unusable but 7.85” is perfect?
No, it has very valid reasons.
First, a 7.85" iPad is significantly bigger than a 7" Android tablet. How much?
A 7" Android tablet display area = 6.04 x 3.54 = 21.38"^2
A 7.85" iPad display area = 6.28 x 4.71 = 29.58"^2
Thus a jump of 38% in display size which is significant. Just because both have the number "7" in the diagonal, it doesn't actually mean their sizes are identical.
Second, 7.85" is important because that puts all the UI elements approximately equal in size to those of the iPhone
as explained here. That's a proven size for touch UI interaction since it's always been used by iPhones, not some random numbers.
I'm still not sure if those UIs are big enough to be completely usable but it'll fill a pretty big niche for many, and 7.85" iPad is significantly different from a 7" Android tablet.