The surface area of the ceramic media may not all be effective surface area. Very small pores may exclude bacteria, or if they are just large enough for a few cells, quickly become clogged with dead cells and debris. A blockage affects surface area interior to it as well as the immediate blocked area by reducing gas and nutrient exchange. So, the effective surface are is likely to be considerably smaller than the advertised figure, helping to close the gap. If the ceramic media were as superior as claimed, you could filter quite a bioload with a single nugget of media.
There is also the issue of oxygen supply. A single nugget is not going to have access to enough oxygen to break down as much ammonia as advertised. Nearby oxygen is not good enough and it takes two parts oxygen for every part ammonia it breaks down. There is also the issue of competition from other bacteria, which also use oxygen but does not break down the ammonia as such. Rather it simply breaks down larger organic compounds to smaller ones, like the dead cells you mentioned. These bacteria, unlike nitrosomonas and nitrobacters, can grow exponentially. So in 24 hours they can multiply more than the nitrifying bacteria can in a month if the food supply is there. Temp and PH also play a role in the rate nitrification can take place with a given colony of nitrifying bacteria.
The surface area at best provides a limiting potential. Actual results vary with too many variables to completely control, but all filter media choices have the same issues to one degree or the other. For ceramic pellets the issues you spoke of is primarily controlled by a very fine prefilter of the water before it passes over the media and other bacteria to remove the dead cells. The prefilter needs to be fine enough to filter microscopic algae.
So yes, you have a point, but when the differential in surface area is big enough and you control for some of the main issues then you can expect to get back a reasonable portion of that advertised difference. Especially since the media it is competing against has some of the same issues to one degree or the other. The fluidized beds Erica mentioned have an advantage that many of those issues are moot by design, with bubblers remedying the main issues not handled by the fluidized bed itself, but there is still the issue of competing bacteria. Especially with lots of larger organic compounds in the tank. Very fine prefilters help a lot with fluidized beds also. Pot scrubbers are a lot less efficient from the start but suffer less from at least some of the variables effecting its efficiency without trying to control them very much.
I think (hope) my upcoming HOB plant filter will be outdo them all in efficiency, ease of use, numbers of contaminant types it can filter (including metals), and the completeness of the filtering process. Nitrifying bacteria only do part of the contaminate breakdown, requiring you to do water changes to remove the unfinished components of the breakdown process. This is a limit of all bacteria based filters.