Is it the fact that there are 2 different sizes; the small free one and the payable mini suitcase one that is causing confusion?
Yes, I think so. EasyJet changed their costings so that if you want to take more than a soft bag which fits under the seat in front of you, you are charged (or have to upgrade the price of your seat or something).
I think it's because people were taking larger& larger cabin bags (for free, rater than paying to check luggage) and there was increasingly not enough room in the overhead lockers for everyone's suitcases.