Most people are aware that at low speeds like 1-2 knots and near the surface, your subs actual depth will be a couple of meters higher than ordered. However, at low speed and deeper depths (like the 100 or so meters you mentioned) the difference between ordered depth and actual depth can be 20 or 30 meters (EX. You order 100 meters, but sub never goes below 80). Both of these are the result of the positive buoyancy feature. I believe the GWX team (Rubini) is aware of the problem at deeper depths and working on a remedy.
See this post:
http://www.subsim.com/radioroom/show...464#post427464
See these other two postings for some discussion on the problem:
http://www.subsim.com/radioroom/show...ive#post394141
http://www.subsim.com/radioroom/show...ion#post394169