APEX FNX 45 MAG Extension +2

The FNX Magazine Base Pad Extension adds two and a half (2.5) rounds to the existing standard capacity magazine for the FNX 45 model pistols. Why is it 2.5 rounds, not 2 and not 3? Apex's mag extension leaves enough space for 2 additional rounds to be loaded, but also enough space for the stack to further compress the magazine spring to allow a fully loaded magazine to be seated with the slide in battery.