Why can the reputation of a user still be 1 in spite of their many posts with high scores? Their reputation even did not changed after I voting up for their answer.
Users can be placed in timed suspension by per-site moderators or SE staff (the reasons for this are varied and as a rule the specific reason is not discussed publicly).
During this period the reputation is displayed as 1; there is also a note on the users profile page that indicates their current status.
Once the suspension ends the reputation is restored (there is no direct points penalty associated with suspension) and the user can keep using the site as before (if they chose to do so).
Another reason why a user can have an unusually low number of points relative to the number of posts are massive distribution of bounties (we had some users that donated away basically all their points).