The alerting service is responsive to state changes, so the alert threshold is a little misleading. Once a state changes, in this case when something goes off of hold on a queue, the alerting service is notified how long the state was maintained. Any and all alerts that fall into the threshold are fired.
I'm assuming the case above, the threshold of 10 minutes was meet firing off all three, while if the threshold was just 7 minutes only two would fire and on down the line.