This difference is mostly about the rock and the technique. Granite is very different from limestone and Yosemite cracks are very very different from a gym. (And Tuolumne domes are yet another thing.) If you spent a summer climbing in Yosemite you'd get comfortable with the style and the grades and they'd make more sense to you.

Conversely if you took a trip to Smith Rock or Owens River Gorge you'd get a much closer correspondence between gym ratings and rock ratings.