Why is the calculated time returned in the __enter__ method using context manager in python?

# timing.py
from time import perf_counter

class Timer:
    def __enter__(self):
        self.start = perf_counter()
        self.end = 0.0
        return lambda: self.end - self.start # Does it not evaluate to 0.0 - self.start???

    def __exit__(self, *args):
        self.end = perf_counter()

With Timer() as time:
    # do something

print(time())

In the above code, according to me, the return statement should be placed in the __exit__() method as it hold the true value of the end time.

I know that the __enter__() method is called once at the beginning of the with block, so it should hold the starting time. Logically it makes sense to me that the __exit__(), which is also called once, can calculate the ending time and then return the elapsed time.

The above code gives me the impression that the __enter__() method is called twice.

Kindly provide me clarity on what is going on.