Some elaboration on Postman's description.
QuickTime video playback in Pro Tools v10 and earlier can be thought of as a black box. QuickTime is the black box. All Pro Tools can do is send video into QuickTime (black box). QuickTime decides when to output the video that came into it. While it can do this in time with a sync input, QuickTime was really designed as a FIFO (First In First Out) process, where as soon as it receives a frame, it sends it to the output destination and then immediately looks for another frame.
Let's revisit the idea that changing your offset by a single 1/4 frame does not make a difference. When you are using a genlocked video card for output, its only concept of time is a whole frame interval (4 quarter frames). While Pro Tools lets you insert single quarter frames (such as 17) for an offset, the video card doesn't understand that 17th quarter frame's position within a sync pulse (whole frame). The end result? Your 17 quarter frame offset is rounded down to 16 quarter frames!
If you have a 19 quarter frame offset, it's rounded up to 20 quarter frames!
Halfway values, such as 18 quarter frames, usually round up to the nearest whole frame.
These issues are supposedly gone with v11 and the Avid Video Engine (AVE). Instead of QuickTime's black box, Avid has more control over the whole process from the moment you press play until the moment the signal is at the card's physical output.