I got an unexpected parsing result when using Forward()

[JacobiSong]

import pyparsing as pp
pp.ParserElement.enable_left_recursion()

# Parser 1
A = pp.Forward()
B = pp.Forward()

A <<= B + pp.Literal("a") + B
B <<= A | pp.Literal("b")
print(B.parse_string("bab"))

# Parser 2
B = pp.Forward()

B <<= B + pp.Literal("a") + B | pp.Literal("b")
print(B.parse_string("bab"))

# Parser 1 output
['b']
# Parser 2 output
['b', 'a', 'b']

I don't known why the ouputs are different.