When a RAM preview begins to play, the first several frames of the video do not play the first time through. Subsequent loops through the same RAM preview duration play all frames. This bug with Mac OS X 10.10 (Yosemite) affects all versions of After Effects.
Previewing a composition is changed significantly in After Effects CC 2015 (13.5.1), so frames are not skipped in during preview.
Downgrade Mac OS X to 10.9, or earlier to avoid this issue.