AMD is not 15.26% share of the video card market as per Steam.
AMD's video card share is higher than 15.26%. I see the source of your confusion.
That's not what Steam says. Remember Steam and JPR track different things.
Steam tracks GPUs.
JPR tracks AIBs (Add in Boards).
These are not the same. Steam doesn't differentiate if the GPU is in a Card or not, JPR tracks only Cards.
As I have been saying all along, the numbers are consistent between them, when you take into account the different things they track.
Maybe you have noticed that gaming laptop GPUs thus far are pretty much owned by NVidia, these are included in the Steam survey, not in the JPR survey.
Get it now?