[racket] Problem with case

Hi all,

Can anyone explain why the first function below selects 'x1-case as
expected but the second fails to select 'x2-case? Am I expecting something
to happen that shouldn't?

Thanks,

Kieron.

****

#lang racket

(case 'a
   ['a 'x1-case]
   ['b 'x1-case]
   [(15 2 3) 'y1-case]
   [(10 11 12) 'z1-case])

(case 'a
   [('a 'b) 'x2-case]
   [(15 2 3) 'y2-case]
   [(10 11 12) 'z2-case])
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