This happens no matter of what cog is set at the rear. But there is a distinct difference in the shifter when this happens as well. When it works OK, the shifter lever can only be pushed one click, but in the cases where it skips the middle ring, I can feel that the shifter performs two clicks - I sort of "click through". I think this is the normal behaviour when shifting from middle to small ring.

But thats why I would like to know why cable tension makes middle ring skip - I suspect that too much cable tension makes it "impossible" to move the shifter further enough to take it to the "last index", so that it believes I'm in the middle ring instead of the large ring and thus performs this "middle to small ring" shift even on the large ring.