I'd think that those 3 shots may be faster due to a slightly smaller pellet. Less friction/drag, means more speed.
Try sorting enough pellets to do a full string and see if it's any different.
Also, if you have never had your HW100 hammer apart, cleaning out the factory thick grease and relubing it with oil can help consistency. Especially in cold weather the grease will thicken and cause erratic velocity.
Since all the other shots look good except for 3, id blame the pellets first. Check them for bent or small skirts, and consistent weight, and head size.