Since I typically leave a few extra tapes in the pool (just in case I have a bad tape or something), the tapes with the higher serial numbers just sit in the library week after week
If I'm not mistaken, it happens due to the fact that backup server gives preference to the expired tapes rather than to free ones. Thus, the said situation.
If you want to leverage all existing cassettes, you might want to increase retention period.
As to slot management, currently we don't have this functionality. However, thank you for the heads up; appreciated.