list() returns different number of comments from a video

I'm trying to crawl comments of a given videoId with youtube API. But the number of crawled comments is less than its actual number. Do you have any idea about this? My code is like the below.

from googleapiclient.discovery import build
from typing import List

def get_comments(api, video_id: str, fields: str)-> List[List[str]]:
    comments = list()
    response = api.commentThreads().list(part='snippet', fields=fields, videoId=video_id, maxResults=50).execute()
    
    all_comment_crawled = True
    while all_comment_crawled:
        for item in response['items']:
            comment = item['snippet']['topLevelComment']['snippet']
            comments.append([comment['textOriginal'], comment['likeCount']])

        if 'nextPageToken' in response:
            response = api.commentThreads().list(part='snippet', videoId=video_id, fields=fields, pageToken=response['nextPageToken'], maxResults=50).execute()
        else:
            all_comment_crawled = False
         
    return comments

api_key = "MY_API_KEY"
api_obj = build('youtube', 'v3', developerKey=api_key)

video_id = 'fgSvGLxanCo'
fields = 'items(snippet(totalReplyCount, topLevelComment(snippet(textOriginal, likeCount)))), nextPageToken'

comments = get_comments(api_obj, video_id, fields)
print(len(comments)) # returns 1,945 actually is over 2,000