Question about Next Numbers by Company

We have next numbers set up by Company - no fiscal year.

It's set up so that ST orders are the same as SO orders.

We just started using ST orders for this company. Even with it set up to use next numbers the same as SO orders, it's assigning the new ST orders numbers that were already used by SO orders a year or more ago. The SO orders weren't cancelled and some of them still have open lines.

Any ideas what could be the reason behind this? I'm stumped.

Thanks!