There are many things to take into consideration here, like how Ichika has less than half of Julia's releases which means she is selling way more copies per title. Then again, you can say that Julia is retired and those are old movies, but on the contrary Julia was marketed as a super star for a long time and posters of her and her movies were all over the place. Can't really say the same about Ichika, she doesn't get her titles on the front pages too often or get huge posters in the entrances of stores.
Julia has also had many years to establish her self, she wasn't star from the get go, she also had a decent amount of surgeries that got her to where she was at the end.
In terms of longevity, that's to be expected, Ichika is playing loli characters while Julia was always somewhat of a milf and we all know for how many decades Julia Ann has milked the milf thing.