2009 IBC Table 508.4

I am having trouble understanding 09 IBC Tab 508.4 which is nearly the same as 06 Tab 503.3.3, at least for my question. Per both tables, no occupancy separation is required between either an F-2 and an S-2 or an F-1 and an S-1 occupancy, however a 1 or 2 hour separation is required between an F-2 and an S-1 occupancy or between an F-1 and an S-2 occupancy.

Could someone please explain the reasoning behind this? It seems counterintuitive to me that a more stringent requirement would be required in the latter case.

Thanks for any enlightenment.
Jerry Stoffel
Building Inspector / Plans Examiner
City of Dubuque.