Yea you're right. I didnt think about the HTC sense UI. So yea basically an update has three levels to pass through until it could get released to us.
Its even more than that with the latest round of Sense handsets though. Because Sense is an integral part of the OS on those phones, even the slightest change in Android could require a significant rewrite.
Combine that with Verizon's excruciating approval process, and you could easily have to wait six months for an update.