Any model in particular that didn't have the tin?
All my models have come in the G tins. My GW-3500 model came in its hexagonal G tin whereas the other models I have each have the round tin.
Where did you buy the watch from that didn't have the tin?
Perhaps the shop had run out and yours was a new model but out on display?
If you buy online, especially from those that provide "free" postage, the tin is often left out to save weight and thus post cost. But some do come in tins and some in cardboard boxes direct from Casio. No particular reason to it