Snap to Guides not accurate

Hi;

I'm a long-time InDesign user and I have come across a problem in one document that is driving me nuts.

I have Snap to Guides on but frames are off-positioned by a very small amount (.062 pts or so). The Snap to zone is set to 4px in Preferences. Other documents seem fine. What am I missing?

I'm using ID 5.5 on MacOS 10.8.2.

Thanks;

Chris