consider following code. I also added a dummy method m to show that precedence is "correct"ly follows for a basic method

from attr import attributes, attr

@attributes
class A(object):
    a = attr()
    def m(self):
        print("A.m")

@attributes
class B(object):
    b = attr()
    def m(self):
        print("B.m")

@attributes
class C(A, B):
    c = attr()

    def m(self):
        super(C, self).m()

    @classmethod
    def supers(cls):
        return cls.__bases__

c = C(1, 2, 3)

print(c)
c.m()
print("supers=%s" % str(c.supers()))

running it results in

C(b=1, a=2, c=3)
A.m
supers=(<class '__main__.A'>, <class '__main__.B'>)

so the first argument is (IMHO) incorrectly assigned to attribute b not a, and then 2nd one to a...

Before recent 3040bda quick and dirty (and possibly incomplete) fix was to remove reversed around mro within src/attr/_make.py but neither I have looked yet into how logic should be adjusted in the current state of affairs, nor I analyzed for why reversed was needed to start with