Let us see this screen (29th November 2013):
I know that Marien..., oh, Donut E. Knot, has many names and this is a source of a strange value. But should I communicate such behaviour of the platform as a bug or think about it as of a pretty joke?
This is not a bug, but accurately reflects the reputation gained over a particular period of time. If you look at the Users page you can select the period of time:
So it's entirely possible for a user who gives a lot of bounties to have a negative reputation when calculated over one of these smaller time periods.