At first I thought you were asking about the viewer rather than when you were shooting.
One of the Frequently Asked Questions addresses this a bit. http://photosynth.net/preview/help#FAQ14
The Expert Shooting Guide also mentions it at the end of 'Spin Tips'
(begins at the end of Page 3, but this quote is from the top of Page 4, just before 'Wall Tips'):
The rule of thumb for turntables is to make anything that doesn’t rotate disappear.
In practice, this
means that any nonrotating background needs to be as featureless as possible."
Unless there is some major difference in how the new Photosynth computes Spins from how it computed Objects in the original synths, it is tracking all identified feature points. This means that it is tracking foreground + background so if your background has lots of unique features, those will match more than the turning object's.