I agree with you. My fastest clocked 400m in high school was 57.5 indoors.
On the other hand, I ran 4:32, 2:03 (only ran the 800 once), and 9:54. For the 800, I came through in 57 (which was a PR if you think about it).
There is no way one can break 5:00 if their 400m best is 60.
But if your 400m is slow, it could be because you simply do not know how to run a 400 (e.g. many distance runners have no clue and begin it like they would a 100m and then burn out).
Ask your coach to put you in some 4x400m at a dual meet, preferably as an anchor, and do one of two things:
1) If you are in the front, make sure no one passes (forget about strategy); or
2) If you are behind, just make an imaginary rope connecting the leader and just real them in (forget about strategy).
Good luck.