[Python] Why does Anagram have a time complexity of O(n+m) instead of O(n)

I was going through few leetcode problems and came across the Anagram Problem. The time complexity for the solution below was mentioned as O(n+m).

We are looking at the length of both strings which should be O(1) and we are looping over len(s) which is same as len(t) => Which should be O(n) and the other loop O(26), => O(1) + O(n) + O(26) would just be O(n).

Why is our time complexity not O(n) and O(n+m)?

class Solution:
def isAnagram(self, s: str, t: str) -> bool:
if len(s) != len(t):
return False

count = [0] * 26
for i in range(len(s)):
count[ord(s) - ord('a')] += 1
count[ord(t) - ord('a')] -= 1

for val in count:
if val != 0:
return False
return True

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