The amount that isn't usable (the "buffer") is for longevity of the cells in the battery, not for balancing. Using the bottom half percent or so of LFP would encourage bad reactions, from poor memory I think it's plating of one of the electrodes. The larger batteries are NMC chemistry, which charge to a higher voltage per cell, so they need a top buffer as well as a bottom buffer. Different bad reactions happen at the top end (high cell voltage). LFP charges to a lower voltage, so it doesn't need a top buffer.
These buffers have nothing to do with balancing. Balancing involves taking some energy from some cells, and either dissipating it or transferring it to other cells, so all the cells become full or very close to full at the same time. Balancing is just as important and effective in LFP batteries as in NMC batteries. Because of the very flat voltage versus SoC curve of LFP, this only makes sense when the battery is near 100% SoC.