brackets in regular expression problem

Can anyone explain the following behaviour?
When I run the script below the results of bracket matches are different:
"1 2" and "12"
------------------------

$_ = '123';

if(/(1|12)(2?)/)
{
  print "$1 $2\n";
}

if(/(12|1)(2?)/)
{
  print "$1 $2\n";
}