That's kind of asinine, really. There are several times in which their powers are presented to be exactly the same. Now, there was a period in which Vixen's powers were inferior to Animal Man's; during 52 when Buddy could pretty much use the power of any animal in the universe. Then again, shortlly after Buddy's powers returned to normal, and Mari's went haywire and started working on other metahumans instead of animals, so it evens out.
Overall, they've always been the same.