+1 To Glenn,
Consider also that a .7" blade is NOT a full .75" and the way measurements USUALLY work is you can go over, but you can't go under. A gallon of gas must be at least a gallon of gas, a 12 oz can must contain at least 12 oz, etc. They can over fill and put 12.2oz in a can, but if they get caught putting 11.8oz in they're in trouble.

So you could apply similar logic when measuring blades today. Particularly when looking at resale practices it avoids a headache to designate a .7" blade as 5/8 instead of 6/8.

Of course if you've ever bought lumber all of this goes out the window...

FWIW
Jim