As far as I'm concerned, if there was such a word as over-interpreting, it would mean that your interpretation contained more meanings than, let's say, a statement actually has, which would make over-interpreting different than misinterpreting.
Let's say that a political speech contains the meanings below;
A, B, C
The misinterpretation of the speech would be; D, E, F
The over-interpretation of the speech would be; A, B, C, D, E, ...