Used Reservation calculation ESXi 5.0.0

Hi,

Wanting to understand the calculation that determines the "Used Reservation" under the Resource Allocation tab of the Default Resource Pool with memory view highlighted.

We have 21 Vm's in this pool and 5 of them have memory reservations of 4096. This totals 20480 but the "Used Reservation" figure shoes 24310.

How is this calculated ? there is a difference of 3830.

Cheers

Brett