I have to laugh. Several bikes ago, I bought a V.O. decaleur without reading carefully enough. I mean, I really screwed up and ordered one meant for 26" wheels and cantilever brake mounts when I wanted a rack for a bike with 700c wheels and road brakes. Rather than try to send it back, I held onto it until I had a project for it. I was excited to put it on my Giant Iguana conversion, but it was off a little too far for me to feel comfortable muscling it into place. Fortunately, Mark Ball (at my LBS) got a fit that works with it. It has a slight angle, but it's close enough.

The point? Sorta what gugie said. These are manufacture using a bunch of presumptions about bike geometry, and if yours doesn't exactly match theirs, some creativity is necessary.

Observation - my OCD is different from the OPs. The angle of the flat of the decaleur doesn't bother me a bit. The curve of the attachment near the fork on the other hand... (hope I didn't set off fresh discontent)