Because the system rounds 5% or less of UTXO down to 0%, and rounds the rest up to the next 10%. So to get 100% UTXO you need 20 BBP * RAC * .901. In User BBP's case that would mean 1,301,683 * 20 * .901 = 23,456,327.66.
This has been discussed repeatedly, and while I personally think the system needs tweaked just a bit, that is the way it is currently coded to work and has been since PoDC Staking began.
I think we should make the steps smaller, like every 1 or 2%.
That way you can get (tiny) rewards with little amounts of BBP and yet require more than 90.1% to get the full reward, in this case 99.1%