You cannot go by comparing the cousin predictions from different websites -- each has their own method of calculating them. Especially when trying to reconcile Ancestry's ranges with other sites. Wait and get them all onto one platform (GedMatch) and run the numbers there.

And to answer a specific question, the way you can match a geadson more than a grandmother is:
1) errors or miscalls in any of the 3 tests (yours and theirs) that can lead to slight mismatching or breaks
2) a small IBS segment next to an IBD segment that makes the total look bigger (highly likely if you come from a population group that is even slightly endogamous)
3) a small IBD segment from one family line and a larger IBD from another family line (i.e. two different shared MRCAs between you and the grandson but only 1 shared MRCA with the grandmother)

Elizabeth