BP to Continue to Monitor Integrity of Oil Cap, AOL New July 17, 2010:
There are a few possible explanations for the lower-than-expected pressure readings. One... reason could be that part of the well apparatus has collapsed and obstructed the oil flow.
It's possible that happened earlier this week, as engineers maneuvered underwater robots to slowly close three valves on a new tighter-fitting cap lowered over the gusher.
Pressure naturally increased as those valves were closed, and could have disrupted some of the machinery inside.