For the same reason that the following results in 100 instead of 00 (0).

$x = 99;
++$x;
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If you exceed the range of the second position, it carries into the third position. To keep it to two digits, you need to take explicit action.

$x = 99;
++$x;
$x %= 100;
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$x = 'zz';
++$x;
$x = substr($x, -2);
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