Quotes vs. no quotes around numbers

[Mark17]

This code works:

if '2011' in baby_names:
    print('Found 2011')
    
if 1 in baby_names[2012]:
     print('Found Rank 1 in 2012')
else:
    print('Rank 1 missing from 2012')
    
if 5 in baby_names[2013]:
    print('Found Rank 5')
 

Why is 2011 in quotes (Line 1) whereas 1 and 5 (Lines 4 and 9, respectively) are not? This trips me up frequently. Thanks!