Since, after scoring a TD, A has no better option that would yield more points, penalties for team B PFs can carry over, to avoid giving B a chance for unpenalized cheap shots.
When scoring a FG, A has the option of going for a TD, so B risks being penalized with 3 to 5 points if A accepts the penalty. So here A has the option of going with 3 by declining the penalty or going for 6+1 (or 6+2).
On a try, A has the same options they have during a successful FG attempt, but additionally they can elect to keep the score and have the penalty carry over, if it was for a PF.
So, on a try:
a) A scores one point, B was offside. A can either keep the 1 point and decline the penalty, or try to go for 2 from the 1.5.
b) A scores one point, B commits a PF. A can keep the point and have the penalty carry over, or try for 2.
c) A scores 2 points, B was offside: decline the penalty, no better option.
d) A scores 2 points, B commits PF: carry over, no better option.