One of the metrics that our creditors use to gauge our ongoing credit worthiness is maintaining and/or beating our ontime departure goals. With so many issues (ATC delays, mechanicals, weather) that can delay flights, when we can get flights out a few minutes early, we will. In this situation, it does not sound like the OP was even on the departure management list, so the GA would have no way of knowing there would be standbys. Also, each UAX carrier has stipulations in their contracts with UA that they maintain a certain level of ontime departures or they could risk loosing their contracts with UA.

Customers have consistently told us that they want to leave on schedule and arrive on schedule. That is what we are trying to provide. If we can leave a few minutes early, assuming all known passengers are accomodated, we will.