Bevel Inaccuracy?

[micheus]

Sorry about the delay...

Now I understood better the object - I was thinking it was in perspective.

No, that is not wrong. Bevel is moving the new vertices to assure that the new edges have the same length.

You can use the Intersect command ([MMB option]) to move the top vertices and align them with the bottom ones.
or, for this specific case, you could select the minor and the greater faces and use Inset command (MMB option and set to [3] Avg Along Edges) to get the result you were expecting to get.