I'm not entirely sure here, but only the RfBs that have passed have garnered over 75% of the vote (there may be an exception or two). The point is, people will be tougher on RfBs, especially knowing the % will be the same. Making it 90% (where did that number come from?) is rather unnecessarily high. I don't agree requirements should be higher for 'crats - as I've stated, their job is easier than being an admin, despite what most people would have you believe.
A crat is also an admin. If they have less community support than would get them through RfA then they should probably be desysoped, not promoted to crat. The percentage support may not need to be higher, but the actual level of community support certainly needs to be.