Block breaking resets when the item you're holding changes

When playing Minecraft I noticed, during the breaking of a grass block, if a block item is picked up, the breaking of the grass block is either reset or "Goes back in time" and takes longer to break.

I expected to be mining grass and collecting dirt.

When I would pick up dropped dirt blocks the breaking of the grass block would reset or take a little longer.

1. Select Dirt in your hotbar.
2. Start breaking grass.
3. As you update the amount of dirt by either picking up dirt or dropping it the block damage will reset.