Say you have a front derailleur cited as having an 18T range, with a 42T main ring. According to the specification, the smallest inner ring can be 24T. But how strict is that? Is it possible to get away with 22T? In particular if the expected use of this ring is very rare: you climb that 25 feet of vertical rock face, and then shift back to one of the other rings.
Similarly for 28 versus 26, et cetera. For example, I have a bike here whose derailleur doesn't go lower than 28T according to spec, and that's the size of the inner ring. But when the chain is on that ring, there is a quite a bit of clearance left between the chain and the cage. I can push the chain down down so that it touches the midpoint of the chain-stay. Then it just about touches the bottom of the cage.