It's not weird at all. There are two types of stale shares in coin-switching business.

The first one is a stale share which is exactly like in a traditional single-coin pool. There was a new coin block and the pool sent a new job to solve. You miner solved an old job at the same time, so it's a stale share. Your miner sends this job to the pool anyway, bit it is rejected as it is trying to solve a block which already is solved.

Coin-switching pools have also a second type of stale shares. When the pool switches coins and sends a new job to solve for a new coin, but there were no new blocks for the previous coin. Your miner solved an old job at the same time, so it's a stale share. Your miner sends it anyway and it is accepted, because its solving a job for a previous coin which is still valid, as no new blocks for that coin were found.

This is why you should always setup your miners to submit stale shares at CleverMining. We are switching coins very frequently and you should always try to submit a stale share, because it can be still perfectly valid for a previous coin. It also can mine a block and earn all of us money just like any other non-stale share can.