If a work interval is N seconds long, TR should report our power for that work interval as our max power over any N-second interval that starts within a couple seconds of the right time (at least if we are finishing the full interval).
Sometimes with Erg mode, we can start and end intervals a couple seconds late or early. With short work intervals, this leads TR to report that we’ve significantly undershot the target power when we haven’t (we just did the work interval slightly later/earlier than prescribed). See for example this thread: I don't get PowerMatch - #33 by bbarrera