It matches becuase the \d{1,3} can match a single digit, then the second digit satisfies the lookahead.

If you run the code with  -Mre=debug you can see it first matching two digits, then matching the lookahead string (thus failing), backtracking, trying to match a single digit, then succeeding:

  Setting an EVAL scope, savestack=5
   0 <> <2005-12->        |  1:  CURLY {4,4}
                           DIGIT can match 4 times out of 4...
  Setting an EVAL scope, savestack=5
   4 <2005> <-12->        |  4:    EXACT <->
   5 <2005-> <12->        |  6:    CURLY {1,3}
                           DIGIT can match 2 times out of 3...
  Setting an EVAL scope, savestack=5
   7 <2005-12> <->        |  9:      UNLESSM[-0]
   7 <2005-12> <->        | 11:        EXACT <->
   8 <2005-12-> <>        | 13:        SUCCEED
                                  could match...
                                failed...
   6 <2005-1> <2->        |  9:      UNLESSM[-0]
   6 <2005-1> <2->        | 11:        EXACT <->
                                  failed...
   6 <2005-1> <2->        | 15:      END
Match successful!
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Dave.