This is because App Engine caches http requests.
I am not familiar with urllib, but with urlfetch from google.appengine.api to ensure you are getting the content fresh from the website you need to set the Cache-Control header, in the example below it will only return a cached page if it is less than 30 seconds old, otherwise it will go to the website to get the page fresh.
content = urlfetch.fetch(url, headers = {'Cache-Control' : 'max-age=30'})