The Oil Drum provides one answer:
Gas flows more easily through cracks than oil, and the disaster was first evident when leaking gas reached the drilling rig, and then ignited. The BOP then, at least partially, functioned. After the rig sank, the riser also sank, bending the pipe just above the BOP. At that time there were reports that a Coast Guard ROV examined the underwater assembly and did not see any obvious oil leaks. A couple of days later the flow was suggested at about 1,000 bd, and this then escalated to 5,000 bd. As cameras began to publicly monitor the outlet of the riser the estimates started to grow, but a not-well-publicized effort measured the flow out of the riser, and found that it was around 8,000 bd, with allowance for leaks, the overall flow was estimated to be perhaps 12,000 bd. Once the broken part of the riser was removed and a cap placed over the well, a significant portion of the escaping oil was captured and could then be measured as it flowed into the surface vessel recovering it. Those values are currently at around 15,500 bd. BP is currently planning on additional capture this week of up to another 10,000 bd, and preparing for a worst case scenario with a flow rate of 80,000 bd. These numbers vary a lot, and yet they could all be correct.
Why? Well, its called erosion, and simply put, the oil and gas that are flowing out of the rock are bringing small amounts of that rock (in the form of sand) out with them. Rocks that contain lots of oil are not that strong and are easily worn away by the flow of fluid through them.