Expected Result:

The Wither's special explosive projectiles (the blue ones) destroy almost any block, since they treat blocks as having a blast resistance of 4. Therefore, they should always destroy 4-5 blocks.

Observation:

Despite the above, the projectiles sometimes only destroy two or even just a single block. This happens especially often if the Wither is very close to a wall and attempts to attack an entity behind it.

Only tested in 20w06a, may exist in prior versions.

EDIT:

Also happens in 1.15.2. Added screenshot. Always only destroys a single block when one of the Wither's smaller heads is stuck inside said block.