Doesn't that depend on whether you ask to stand by before or after you check your bags?

Lets say I checked a bag in BOS for BOS-JFK-JAX and arrive at JFK at 2PM. My JFK-JAX flight departs at 3:30PM, but there is an earlier flight that leaves at 2:40PM. If I ask to standby on the 2:40PM flight, would they have time to pull the bag to load it onto the earlier flight?

Although the times above are fictional, it is a true story based on my attempt to stand by for an earlier flight. The agent at the Just Ask desk said that although I would be able to make the flight, my bag would not arrive until the later flight did.

I'm assuming that if you check a bag for a booked flight, then standby onto the earlier one with very little wiggle room before boarding, they're probably not going to track down your bag to pull it.