My experience at SFO is just like yours. Never inspected before GE, about 30% customs inspection rate after GE. I now fill out the regular immigration form on the plane, and if there's no line, I go through immigration and skip GE. I've never been inspected by customs when I do that, so this technique would seem to get me out of the airport faster.

On the other hand, there could be some rational explanation that we're not aware of, like "higher rate of illegal imports at SFO" or "US-wide increase in the number of customs inspections"...