Comps have their own way... Round 2 in Heusenstamm

After 3 of 7 rounds at the Heusenstamm Sparkassen Open I am pretty satisfied - no real mistakes and a decent game against a 2300 guy: I managed to keep the game within the higher end of the drawing-range with a minus on my side of 0.3 to 0.45; juuuust enough to survive. Here´s the final position:

(White sacrificed the exchange some 3 moves earlier - in order to escape the draw.)
Black´s last move was Rxa8 - offering a draw; which was accepted by White.
When I entered the position into my comp - the machine gave Black a decisive advantage of +2!
I was shocked - believing that I made a big mistake by offering the draw (too early)!!!!  But then I kept making the moves suggested for Black (and ignoring the suggestions for White) for some 20 moves - the position as such didn´t really change, but the machine keeps his evaluation error at +2...   strange...