join omitting output lines when input sorted numerically

i have two files, aa and bb:

 $ cat aa 
 84 xxx
 85 xxx
 10101 sdf
 10301 23
 
  $ cat bb
 82 asd
 83 asf
 84 asdfasdf
 10101 22232
 10301 llll
 

i use the join command to join them:

 $ join aa bb
 84 xxx asdfasdf
 

but what expected is 84, 10101 and 10301 all joined.
Why only 84 has been joined?

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