Думаю, вам нужно следующее решение:
import random
aa = []
for i in range(10000):
aa.append(random.randint(1,100))
avg = (sum(aa)/len(aa))
abv, blv = avg +10, avg-10
final_list = [1 if i>= abv else 2 if i<=blv else 0 for i in aa]
print ("numbers above, average+10 = ", abv , " --> ", final_list.count(1))
print ("numbers below, average-10 = ", blv , " --> ", final_list.count(2))
print ("numbers above, between(+10 and -10) = ''", blv, ":", abv , "' --> ", final_list.count(0))