It's not a medical issue. Some kids have small bladders and some are heavy sleepers. (and you've probably heard of both of those) What you may not have heard of is that we all have an antidiuretic hormone that is released at night that causes the kidneys to release less fluid. Some kids don't make enough of this hormone. This doesn't mean they won't eventually make enough. Their bodies just haven't gotten there yet.
I would be concerned, however, if she were dry at night for several months and then all of a sudden the bed wetting began again. If she has constantly been wetting the bed (maybe a few dry nights here and there) I think you just need to wait it out. Ask your pediatrician about what I've mentioned.
Trust me, I understand your frustration. I also have a 9 year old third grader who wets the bed almost every night. We don't even deal with wet sheets. She used to wear pull-ups at night, and when she outgrew them she went to Good nites. I admit, I am a little worried that this isn't going to stop before she outgrows the good nites. And then what? Depends? But I know it's not my daughter's fault. What 9 year old doesn't want to wear underwear and be dry at night?!
I wonder if she would stop lying to you about it if she knew you weren't upset about the bed wetting? When my daughter gets upset and asks why this keeps happening I just try to tell her her body will tell us when it's ready. It is what it is. I hope that helps a little, knowing there may be a reason for it, and knowing other people are going through it too!