Low Leak Rate - Please comment!

All,
I am using the Bipap Auto w/ biflex, set at Epap 5, and Ipap 20. I generally get a leak of 25 lpm on average using the Resmed Activa mask and chin strap. My 90% Ipap at this rate was 10 cm. Last night I had a 20.37 lpm leak rate at Ipap 90% of 9cm.
Until this happened, I had not read closely the pressure flow curve chart for my mask. The flow chart says at 9 cm my flow rate should be about 29cm. Also , I understand that my bipap pressure (EPAP) will be as low as 5 cm. The leak rate for 5 cm is about 21 or 22 lpm. This does not explain the apparent low flow. According to the mask flow chart I would achieve 20 lpm leak at 4 cm. I have checked my mask and don't see an problems, such as the vent hole clogged. Perhaps the masks vary. Please give me your thoughts.

P.S. This forum has been the best source of apnea info I have found and my thanks to the many folks who take the time to offer their experience to help others.

I am the guy who posted the above. Somehow I go logged out before I finished the post.
curtcurt46

Curt, I'd be ecstatic that the leak rate was nice and low. I wouldn't worry a bit about it. I don't know if it would even be possible to zero in on the variations you're looking at, given that the machine is going to vary the IPAP/EPAP throughout the night.

The 90th percentile pressure doesn't mean that one pressure was used during 90% of the night. It means that pressure OR pressures BELOW it were used 90% of the time.

You've got a wonderfully low leak rate...I wouldn't worry a bit if it were somewhat lower than expected sometimes. An exhaust could get partially blocked occasionally by bed covers or the pillow, but I wouldn't worry even about that.