What is the Blunder?

After your move you can be hit with 1, 2, 3, 4.
After the correct move you can be hit with 2, 3, 4.

The concept of duplication says: Move your blots in such a way that they can be hit with the same roll. This reduces the probability of hits and double hits.

Thank you. It appears to be a large penalty for such a small difference.

A secondary factor - after the correct move, if your outfield blot is missed, you can cover your blot on the 9 point with a 6. After your move you need both dice to cover. This is only a minor factor - after all you aren’t really trying to make the 9 point. But in variations where your opponent points on your inner board blot and starts to get a blitz going, buttoning up your position can be valuable. If he does succeed on his blitz, having one blot is much better than having three. There is also value in not being hit if he tries to run his last checker. You get more return shots when you don’t have a checker on the bar.