PCQ fropping packet for no good reason on RB750

Have anyone noticed PCQ dropping packets or unnecessary queuing even though the packet rate is small, way below any of the limits (even limit-at)? Cpu is below 20%, queue size is 50/300, 1-3 PCQ streams.

I see it on RB750 with ROS4.11 and ROS4.10 (150 HTB queues) but I don’t see it on RB493AH ROS4.5-same traffic, same rules, queue sizes etc.

Update: I upgraded RB493AH to RB4.13 and it’s still fine. Therefore it seems that only RB750 has this problem.

I talked about it - http://forum.mikrotik.com/t/queues-something-wrong-with-speed/39015/7