There's some slightly different calibration on the CRP5 isn't there? Something to do with compressibility ... anyway I remember when I was doing the ATPL flight planning I was trying to use an E6B and kept coming up with an answer that was a few knots out. But when I got hold of a CRP5 it seemed to solve the problem. God knows why one costs more than the other though, they're just little round plastic things, must cost all of about 5 p to make the bastards!!