Average function without sum() BUG

Question

I'm a beginner and currently doing the "Programming for everybody" course of the University of Michigan.

I wrote this code myself for task 7.2 and to my understanding the code is basically done. But for some reason at line 7 of the output (you'll see when you start the program), the program starts adding 11 additional digits at the end of the sum, as if I just substracted "0.00000000000000001" from the sum.

I just can't wrap my head around why this happens, does anybody of you see the problem?

The text-file used for this can be found here: https://www.py4e.com/code3/mbox-short.txt

Thanks a lot.

file = ("mbox-short.txt")
text = open(file)
lcount = 0

for line in text:
    line = line.rstrip()
    if not line.startswith("X-DSPAM-Confidence"):
        continue
    lcount = lcount + 1
    print()
    print("Line: ", lcount)
    #print(line)
    a = line.find(":")
    numbers = line[a+1:]
    numbers = numbers.rstrip()
    numbers = float(numbers)
    print("New Number: ", numbers)

    if lcount > 1:
        print("Old sum: ", numbers_sum, "+", numbers)
        numbers_sum = numbers_sum + numbers
        print("New sum: ", numbers_sum)
    else:
        numbers_sum = numbers

numbers_avg = numbers_sum/lcount
print(numbers_avg)