When you open removable storage like an SD card on a computer, and you delete something, both Windows and Mac will by default send the deleted items to the trash.
When something is sent to the trash on Windows or Mac OS, it's not actually deleted from the storage device, it's just moved into a hidden .trash folder, and thus hidden when viewed on the computer. It is still present on the card though, and these files are not hidden from the SD reader on the mixer.
If you have deleted old sessions but they are still showing up when the card is in the mixer, you should at the very least put the card back in the computer, and then empty the trash. That should actually remove the deleted sessions from the card.
Due to the nature of flash memory though, depending on the size of the sessions deleted, the size of the card, and the amount of used space, this will only work for a short while before the performance of the card degrades, so ideally, you should do a full format of the card using the SDCard.org format utility, per this article:
http://support.presonus.com/hc/en-us/articles/115000236783-What-utility-should-I-use-to-format-my-SD-Card-
