Adding 1 to 99 and getting 100 shows that we're not incrementing in string land

Actually, you didn't add 1 to 99 to get 100, you incremented the string '0099' and got '0100'. This is indeed incrementing in string land. String incrementing is magic and uses the ranges of digits, lowercase alpha, and upper case alpha to do the right kind of roll-over for each slot in the string.