I'm also not sure I'd get too hung up on figuring out what works for you with the Bontrager saddle. Depending on how your weight is distributed on each bike, the saddle is supporting your weight somewhat differently. I'd venture a guess that you sit slightly more upright on your Trek, which puts less pressure on your soft tissue. If you put the Bontrager saddle on your Surly, it could feel very different to you.