G1 calculates short term pause time ratio by

ratio = (current pause time * X) (time of current end of gc - current-Xth end of gc)

where X is the current length of the queue containing the last X end-of-gc pause times. (This is to smooth out spikes in collections that are close together).

The problem is that G1 pushes the current collection end time *before* this calculation onto that queue, which means that the denominator in above calculation uses one less period than required.