Inconsistent buckets are the processing industry's term for, "there's no way to compare credit card processing quotes that are based on tiered pricing."
Tiered pricing allows a processor to manipulate charges behind the scenes. Essentially, they can raise your cost without having to raise your rates. They do this by routing more interchange fees to the mid and non-qualified pricing tiers. Since there's no consistency regarding interchange qualification, it's impossible to compare tiered pricing among different processors.
Let's look at an example to illustrate inconsistent buckets. Let's pretend that we have the following quotes from two different processors:
Qualified Rate: 1.49%
Mid-Qualified Rate: 2.59%
Non-Qualified Rate: 2.99%
Qualified Rate: 1.69%
Mid-Qualified Rate: 2.25%
Non-Qualified Rate: 2.49%
Look only at the qualified rate, Processor A is offering a much better deal. What you don't know is how many interchange categories are being routed to the qualified tier. Processor A may be routing much of transactions to the mid and non-qualified tiers making Processor B the better option. Of course, there's no way to tell just by looking at the numbers.