I would have initially thought the sabot/bullet too. But with the 5th and 6th shots shooting only slightly lower than 1 and 2 I would have to think the lower shots were due to barrel expansion.
My reasoning is 1 & 2 were high. Quickly followed by 3 & 4 which by this time would have warmed up the barrel making a large difference in barrel temp and sabot/bullet. The only thing I don't know to confirm my theory is how long after #4 was 5 & 6 fired.