The braisa lists several halachos which are calculated based on mais lais (year to date) and not according to calendar years – minyan olam – which Rashi explains is referring to (the year beginning on) Rosh Hashana.  However regarding the view of Ulah that the halachos of nineteen and twenty are calculated not according to meis lais but rather according to minyan olam, Rashi explains that to mean merely that within his twentieth year (not Rosh Hashana based) it is sufficient to be considered twenty years old.  It seems that in this case Rosh Hashana is not part of the equation at all – what is the difference between the two?