It starts like this.
One has to assume that new phone needs at least a six month time to product cycle. I can assume HTC and Samsung can be very fast at that (not like Nokia or Blackberry) that a three month time maybe possible.
Given this cycle, the phone is likely to have the latest OS at the time when it was conceived. You can definitely say the Thunderbolt, Inspire, Atrix, LG Optimus 2X had conception dates that predate the introduction of Gingerbread, the date being right about the time they change the statue in front of the Google campus. The Galaxy SII had a much later conception date, probably about the time Gingerbread was already released to the makers.
The Thunderbolt as well as the Aspire, are versions of the Desire HD. The Desire HD was launched in September of 2010 but shipped around early November. Froyo sculpture appeared on May 20, 2010. I would assume Desire Z and Desire HD development started soon after that.
Gingerbread statue appeared on Google A4 building on October 23, 2010. I assume that Galaxy SII development was given the greenlight around November.
Why didn't Thunderbolt shift to Android 2.3 before launch? If they did that it would have delayed the product launch because the update requires passing a lot of tests before given approval. Better not to tie down the product launch this way or your customer (the carrier like Verizon or AT&T) would seriously get pissed. Besides, when orders are made, it will be black and white. The specifications are fixed, including the OS version, in black and white and so is the delivery date.
I can imagine that sometime around November, HTC showed a Desire HD to Verizon representatives, and the Verizon people might have said, lets make a 4G phone out of this. Give HTC a little time to add LTE and so on.