If you see a reservation available for a short timeframe, it means another unit has split off that portion of the reservation and released it. This is done manually be a unit (auto release never splits off individual shifts automatically).
There are two ways to confirm:
You can usually tell this is the case by looked at the Scheduling agreement. If the agreement shows 4-8 is the agreed on time for a Friday night shift and you see a reservation for 6-8, that means another unit had 4-8 and released only the 6-8 portion.
You can see reservations that are claimed by other units under Storefronts > Create my Own > View Trail's End Storefront Reference List