Why are there both % and $ variance limits for Bill Approvers?

Answer:

Whichever variance is exceeded first will be the first blocker. Each variance behaves independently of the other. 

For example, there are two approvers in this Bill approval routing. The first approver is %=10%. $=$50. The second approver variance is infinite for both % and $ because it's the final approver.  The Bill's original amount is initially $100. The Accounts Payable user changes the Bill amount to $140 and submits the bill to the first approver. It exceeds the 10% limit, but it is lower than the $50 variance. This bill must go through the second approver because it exceeds one of the two variances.