How can two dogs from the same litter, trained by the same person, be so different? (please read details)?

We have had these dogs from birth. They are blue heeler, black lab mix. They are now 3 years old.

Spot listens to me. Bear doesn’t.

Example- I let Spot out to pee. I call for him. He comes back.

Now it’s Bear’s turn. Bear has been gone for 20 minutes now. He won’t respond when I call for him. (We live on a 200 acre farm. He’s either roaming the fields or about to get hit on the highway)

:/

Help!