I don't have my alt cover and motor handy to look at, but I had the same problem. You are installing it wrong.
Isn't there another tab with a rubber boot? So there are 3 points that need to be put in correctly. I think you are installing it wrong, it took me 3 times to fit it right.
I think you are correct.. I looked more closely and there is a tab on the center left that fits into a slot that does not seem to be aligning properly.. I will check that out tonight.