Um, yeah, Abby does that, too. She also amazes me that she's able to go so long with out peeing. For instance, she was dry overnight & then didn't pee until about 10am this morning. She throws a fit when I want her to sit and "try". So I do try to get her to try, especially when we're out & about, but it doesn't always work - no easy task to try to wrangle a toddler onto a potty if she doesn't want to go. ;) She always does make it to the potty when she does have to go, so I'm kind of just going with it.
Sounds like we're in the same situation. :)
Sometimes I'm able to coerce her into "trying" if I give her a choice like "Let's go sit on the potty before your nap. Should we read the Spot book, or Curious George?" Again, sometimes letting her make a choice totally works, and other times it's still no way.
Don't know that this helps, but FWIW, that's what's happening here.