Hmmm... this presupposes that PRS isn't actively selecting for higher grades of trees in the first place....
EDIT: As garrett points out below, "10 Top" doesn't mean top 10%, it means a "10" in quality (on a presumed scale of 1 to 10 with 10 being the best).
From the PRS FAQ:
"A very small percent of the maple trees cut in North America are actually figured. To make "10-Top" status, a PRS top must have clearly defined figure across its entire top with no "dead" spots. A guitar designated as a 10-Top will usually have a small "10" written on the back of the headstock in the upper right corner. Some of our earlier guitars have the "10" stamped into the finish in the same position. "
But, PRS is just that good at wood selection. Their buyers have years of experience sourcing wood. The fact that a disproportionately high percentage of PRS guitars end up as 10 tops or better means that the buyers are simply passing up the lesser woods.
So, as a whole, there are few well-figured trees on the market, but PRS predominately selects only those trees.
This also means that "Artist Grade", "Signature Grade" and "Private Stock Grade" tops are better than "10" on the scale.