The a == b expression invokes A.__eq__, since it exists. Its code includes self.value == other. Since int's don't know how to compare themselves to B's, Python tries invoking B.__eq__ to see if it knows how to compare itself to an int.
If you amend your code to show what values are being compared:
class A(object):
def __eq__(self, other):
print("A __eq__ called: %r == %r ?" % (self, other))
return self.value == other
class B(object):
def __eq__(self, other):
print("B __eq__ called: %r == %r ?" % (self, other))
return self.value == other
a = A()
a.value = 3
b = B()
b.value = 4
a == b
it will print:
A __eq__ called: <__main__.A object at 0x013BA070> == <__main__.B object at 0x013BA090> ?
B __eq__ called: <__main__.B object at 0x013BA090> == 3 ?