Usually when we reserve a spot on the plane (there are limits to how many in-cabin pets are allowed in each cabin) for our underseat dog, the reservation agent requests the carrier dimensions and checks a chart of allowed under seat sizes for each type of aircraft and for each cabin for those planes to see if the carrier will fit per DL's list of allowed carrier sizes. For soft sided carriers there is more wiggle room since the carrier can squash a a bit but hard sided carriers can't squash and can be an issue if space is tight. In this case someone messed up by allowing a passenger with an in-cabin pet book a bulkhead seat which obviously has no under seat room for a pet carrier. A big limit on underseat space turns out to be the electronics bay for the IFE since they are mounted under the seats.