Well, the delays usually pile up during the busy hours. They manage to eliminate the delay times when the flights arrive or land at the non-busy hours which dissipates most of the delays. One or two flight cancellations won't have much impact on the departure or arrival time during busy hours because the airport is packed.
This situation is very similar to rush hour traffic on roadways dissapating after those hours.
To answer your second question, it is mostly airlines that cancel their flights. Airport management might cancel a flight if there is something really wrong with it. and ATC might cancel if the weather or some other occasion warrants it.