Yer pays yer money and takes yer chances ... while the answer may seem flippant, it's not.
All engineering decisions involve compromise to some extent, whether that be large or small. No different with a trimsheet.
If one wants a maximally flexible and detailed trimsheet, it is simple enough to design. The advantage is accuracy at the expense of taking longer to fill it out, etc.
If, on the other hand, the loads are pretty predictable so that the errors and error analysis need not be too restrictive operationally, then the calculations can be dumbed down to increase the speed of sheet completion and apparent operational simplicity. The penalty in the case of the present sheet is that
(a) there will be company prescribed requirements regarding seating distribution to minimise the error limits on the sheet
(b) the cabin staff will, most certainly, have to ensure that the design presumed pax distribution is reasonably met. Typically, this will involve each of the two zones having either a generic desired loading distribution or a specific seating order. Generally, it is only necessary to make sure that the distribution is reasonably even and the error analysis in the sheet's design takes care of the maximum expected errors.
If the above is not practicable .. then you go for an increased number of loading zones. All a case of compromise between accuracy (and minimising error restrictions) and operational flexibility.