IF you have 10 mythic mounts then you get 6000 added to your item level instead of 3000. This appears to be because equipping a mount combat power adds 3000 to your item level but without any ratings, thus reducing your ratings by 3%.
By comparison the companion deals damage but only adds it's item level once, shoudn't the mount do the same? This isn't a problem in adventure zones as IL = damage but in scaled content upgrading your mounts reduces your ratings.