AS poor staffing means allowing more time to process boarding and close the flight. AA, DL, and US all have T-15 boarding for domestic and manage it. But, for mainline flights, there will typically be a "floater" GA who arrives to help with standbys, upgrades, gate checked bags, and general whining.

The baggage check for AS is consistent and, as others note, at smaller stations, the same staff handle multiple functions, so the cut off may be absolute.