I don't know to much about what the rules of dispatching are, but coming from the other end... I'll take a stab at it.
The computer recommends units based on distance, priority, and availability. At times all units within a region, even a entire borough can be "busy." Manhattan is split between, South, Central, and North. Bronx units get sent to Manhattan North, Queens to Central, and Brooklyn to South all the time b/c of the high call volume in Manhattan.
Now Staten Island would generally be closest to the South. If all Manhattan units were busy at the time; all units bordering Manhattan in their respective boroughs were ALSO busy, than there would be a possibility of a ridiculously long response time.
Now here lies the problem. If a closer unit becomes available seconds after the job has been given out, I've noticed that the dispatcher does not always catch it. He/she may choose NOT to reassign the job, instead giving the available unit yet another job.
Seeing that the recent weeks have been extremely busy, this may have been the case.
Last Thursday I happened to be doing some BLS overtime in Midtown Manhattan. If I remember right, I heard the SI crew go over the air several times stating their location, the fact that they were in bumper to bumper traffic in SI. The unit tried to refuse the job, stating that there was no way they could make it to midtown in a sufficient time. They had been refusing to press their "63 enroute" button. A few moments later, I heard a LT or CPT go on the air and basically say that if the unit refuses, put them out admin and send them back to their station. The crew ended up responding to the call since I didn't hear anything from the dispatcher.
That same week I was in downtown Manhattan around 16th street on the east side. I was dispatched to 98th street on the west side for the Cardiac Arrest. Dispatch was notified of my location and it was documented, but we continued in. The BLS, ended up transporting the pt on their own b/c of our asinine distance at the time. Yet there were several ALS units that became available as I fought my way uptown.
Can we question dispatch? Not really.