What is meant by "guarantee" is what is confusing you, I think. When most people make a reservation, they are assigned a specific stateroom, and it is what they will get. These are the people who do not have a guaranteed category.
"Guarantee", in cruise parlance, means that they are guaranteed to have at least that category of which they've purchased the guarantee, but may in fact end up with a better category. When you book the guarantee, you don't get a stateroom assignment, but will be assigned a room close to the sailing date. So, you're guaranteed to have that category, but you can't choose your room, and have to take what they give you. Basically, I think you get what's left after all the other bookings are made. On occasion, they'll sell all the rooms in one category, and you could get upgraded, but I'm not sure how often that happens. I think that guarantees are somewhat less expensive, but how much less probably varies from line to line and also depends on the sailing date.
I hope this was somewhat helpful....