Beautiful Soup findAll doesn’t find them all

Different HTML parsers deal differently with broken HTML. That page serves broken HTML, and the lxml parser is not dealing very well with it:

>>> import requests
>>> from bs4 import BeautifulSoup
>>> r = requests.get('http://mangafox.me/directory/')
>>> soup = BeautifulSoup(r.content, 'lxml')
>>> len(soup.find_all('a', class_='manga_img'))
18

The standard library html.parser has less trouble with this specific page:

>>> soup = BeautifulSoup(r.content, 'html.parser')
>>> len(soup.find_all('a', class_='manga_img'))
44

Translating that to your specific code sample using urllib, you would specify the parser thus:

soup = BeautifulSoup(page, 'html.parser')  # BeatifulSoup can do the reading