Pure guess, because I have no idea what the Poker Tracker SD is measuring, nor do I have PT.
A single hour might not give you a decent sample, just like 100 hands is not as good as 10,000 hands at determining any particular measurement.
If it is measuring your play, you might think about taking random hours, perhaps over multiple days or sessions, say 10 of them, and recalculating those values to get a more valid idea. Once you determine a baseline with sufficient sampling, any particular hours SD will give you better info.
If it is measuring opponents play, that might be hard to do.