Probably you have taken actions that create new slots, like moving an attachment to a new bone. When you have slots hidden, you won't see that is happening.
Please read here:
Basic Concepts - Spine User Guide: Slots
Slots allow you to group attachments which will all appear in the same place in the draw order. This reduces the number of entries you need to manage for the draw order. For example, all swords can be under a single sword
slot. In the draw order you just place the sword
slot where you want it
you don't need to place all the swords individually (there could be hundreds!).
You may wonder if bones can do this job, but they cannot. It is common to need multiple attachments on a bone, but have them in different places in the draw order. The link above gives an example (belly, shirt, pants).