This was a bit of miscommunication on my part. :x Cho'Gath R has the same selection weight as a rather large number of other ultimates.
When UltBook was initially released, ChoR was had extra weighting in favor of a specific set of champions. As does most ultimates, there is slight weighting towards making sure your selection has 1 thing playable in it.
The change of weighting to ChoR was to remove that special weighting. Meaning that champions it had been weighted in favor of before will get it less (and instead it will show up about the same as, well, most other ultimates for them). But also that every other champion will get it more. Without special weighting, effectively every champion has an equal chance of ending up with ChoR.
The net result is that there's overall fewer ChoRs in the game than in that initial release. But the difference is small, matches it to plenty of other ultimates, and for many characters you actually see it slightly more often (such as on most ranged champions).