For the charts we use anyway, whenever they moved from MDA to DA on NPA plates, literally all they did was change "MDA(h)" to "DA(h)". The values never changed and therefore there is no allowance for dipping below that value in the go around like there is with an ILS "DA". For this reason we add 50' to cover this scenario. I imagine the chart creator doesn't want to add this value themselves to make a 'true' DA because they value to be added really varies depending on the aircraft - momentum suggests that a C150 going around is unlikely to lose the same altitude as an A380.