It could be if the front derailleur height was adjusted based on its position relative to a part of the chainring that wasn't its tallest section. The chainring on my bike isn't oval, but its not perfectly round either. When on the lower ring, it is the big ring that strikes the derailleur cage.

If that is the problem raising a derailleur a few millimeters should fix it. How you accomplish that would vary depending on whether your derailleur is a direct mount or band-on type.