I have seen this also. Yesterday in my geame there were 35 roman provinces, 35 Egyptian provinces, and 35 druidism provinces. They all had vast differences in power, with druidism the highest (and Roman the lowest). If this is WAD, I'm curious to know what the design is (it is more than temples and spices to be sure).
Also there are 100+ Greek provinces. To compete with that is impossible, because the spread rates for religion get so low for larger empires - base MTTH for 50+ (for Romans only, apparently) is 200+ years. Not sure if that is WAD either. Seems ridiculous when there are hardly any size penalties on culture spread. Of course, culture indirectly gets hit by the religion, because its MTTH is doubled with the wrong religion and halved with the right religion.
It seems like religious power is generated over time. If that's the case it seems like it'd make sense for at least the roman religion to spread fairly quickly as it gets bigger so they can "catch up".
As it is it seems impossible for the Roman religion to get much omen power, unless I am missing a major strategy point (if so, tell me). That seems bugworthy.