you know it's funny as I actually looked up and read the ASTM 681 (?) spec sheet for precision ground tool steel as I wanted to understand what that tolerance meant. As far as I could tell it's a "to specifications" tolerance ie. if the piece is listed as 0.750" then it has to be within 0.749 and 0.751". It's not a paralellism tolerance or a "by length" tolerance, so it could be wavy by 0.002" all the way along and be in spec. I would guess though that what it really means is that if you order a 0.750" wide piece, it will be 0.749 or 0.750 or 0.751 all the way along the piece, but if you bought a second piece they only guarantee that it would be within 0.749-0.751, not that it would be the same width. As you're buying one piece and cutting it into smaller pieces, that doesn't matter.
The Amazon listing has ASTM 681 in there, so my guess is that the width tolerance they listed is an error or for larger widths.
As I said earlier, you need to be patient if you want to get the good prices. If you need it right now that can bump the cost. For me the total came to about what a cheap import set of 6" parallels costs, but I have more confidence in the parallelism and matching of the ones I made.