Do not forget that when you activate the app phase, the FMGC computes speed at present weight (for ex 63.2 t) then when you do the go around, your weight will be different maybe 300 or 400 kgs depending when you activate the app phase. So your speed will be slighty different (1 or 2 kts) no more.
The other reason is that the value of S F O speed might be slightly different (for ex 1.22 to 1.25 of Vs) depending of phase