So my question is: what is a guaranteed cabin?
Let's use Carnival as an example. When you book a guaranteed cabin it is for a specific class of cabin - outside, balcony, inside. What this means is the cruise line is guaranteeing that you will at least be in the lowest category for the cabin class you picked, but you could be moved to a higher category within that class. So if you booked "OV" this means you picked ocean view cabin, and you will at least be in category 6A but could be in 6B,6C, or 6D. Some ships also have category 5A which is portholes and not a window. Of course, all of these cabins are otherwise identical but just on different decks. The guarantee is for a cabin within that class and not a guarantee that you will be moved to a balcony cabin or suite. You are paying less and so could be in a category 6D cabin for less money than someone who specifically paid for a 6D. It could be as little as a $40 difference but I've seen the price difference from OV to 6D be $150 or so. But chances are when you book a guarantee you will be put in the lowest category for that guarantee.
Some people seem to think cruise ships have endless supplies of empty balcony cabins and suites just waiting for someone who booked minimum inside to be upgraded.