Comparing BP flow rates is flawed thinking for at least two reasons. For the first 40+ days, the flows were coming from three partial holes in the pipe, not a completely open stub.
Without the riser, water pressure was holding back the oil flow. With the riser, downward water pressure pushes the oil up the pipe towards the surface and BP is likely pumping the oil to create a vacuum to reduce the pressure at the cap so it leaks less.
