Did some sleuthing, and I actually think it’s the latter. If you search for gift cards for either company, each denomination has its own store listing with a unique ASIN*, but with the functionality of changing the denomination from within the listing.
So either Xbox is selling $100 gift cards at a rate that puts them in third place on the top 10, or people are clicking on that listing, but changing the denomination before they check out. Unless something happens on the back-end, it seems the ASIN stays the same even if you do change the denomination.
It seems unlikely to me that a significant number of people would first choose the $100 listing and then change it, instead of just choosing the proper listing the first time around. Especially since a simple search of “Xbox gift card” doesn’t put $100 at the top of the search, but rather in the fifth position.
So, yeah… It would appear to me that Xbox players receive way more generous gifts than either PlayStation or Nintendo players. Which is weird, and makes me wonder if I’m overlooking something. Or maybe it has to do with their differing demographics.
*ASIN = Amazon Standard Identification Number