"Optimizing Queries" video (Udacity web dev course), caching db queries

Here is the video that I am confused about: https://www.youtube.com/watch?v=9UHKsiDznwg

At about 6:12 you can see the code he uses to cache database queries. What I don't understand is why he uses a dictionary to cache the queries. The way he has it set up won't the dictionary only ever store one value at the 'top' key that he sets. Why doesn't he just store it in a variable? I have included the main code in question as well.

CACHE = {}
def top_arts():
    key = 'top'
    if key in CACHE:
        arts = CACHE[key]
    else:
        logging.error("DB QUERY")
        arts = db.GqlQuery("SELECT * "
                           "FROM Art"
                           "WHERE ANCESTOR IS :1"
                           "ORDER BY created DESC"
                           "LIMIT 10",
                           art_key)
        arts = list(arts)
        CACHE[key] = arts
    return arts