After doing a little digging I found that when shooting in 24PA mode, the camera flags certain frames for removal. Specifically frame numbers 00 01 02 [03] 04 05 06 07 [08] 09 10 11 12 [13] 14 15 16 17 [18] 19 20 21 22 [23] 24 25 26 27 [28] 29. Then FCP removes those frames [the ones in brackets], which results in the removal of 6 frames per sec.