Time left on remaining battery capacity is calculated based on actual power consumption.

Meaning there's less on-time in a battery when power power consumption is high.

Though the differences seem large for just 5 secs apart.

I see the same in a D610, though with less steep variance

and it doesnt always change every 15 or so seconds its more at random times

well i may have exagerated the 5 second thing its  more like 15 but still

btw my old battery was a 6 cell and this one is a 9 cell i dont know if that would make a difference or not