Continuous Scroll w Playhead Problem

[Count Zero]

When scroll is set to playhead and keypad is set to shuttle in operation prefs my edit locations change when i shuttle with the pad but they dont exactly update to the playhead location. This happens regardless of whether "link edit and timeline " is selected.

Have I

1. got a corrupted session

2. misunderstood what is supposed to happen when I use the shuttle with playhead

3. found an "issue"....

When continuous scroll with playback is selected the edit marker updates to the exact spot when I shuttle......

As I am relatively new to pro tools and have only just got into using some of the more advanced tools I am a bit confused....