Choice rule not choosing

The two answer set programming (ASP) programs below are different.

Program 1

1{a(D):b(D)}1.

b(x).
b(y). 

Program 2

1{a(D)}1 :- b(D).


b(x).
b(y). 

To understand their differences, see their answer sets respectively.

For program 1

Answer: 1
b(x) b(y) a(y) 
Answer: 2
b(x) b(y) a(x) 

For program 2

Answer: 1
b(x) b(y) a(y) a(x) 

Now I think you have figured out. The condition in choice rule plays a role.