If a sticky piston is powered for just 1 tick, it will retract without pulling the block that it pushed. If you connect it to a rapid pulser, the sticky piston will never pull a block immediately after pushing it. So the block of dirt will move only half as often as the piston head does.
This happens in creative mode, but I'm not sure if it happens in survival mode.
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Duplicate of MC-5726 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.