weren't penalty tries a possible 8 points? You'd get a kick from where it was scored and then one in front?
A Penalty try is awarded when a try would have been scored had it not been for an illegal act that would otherwise be penalised. ( Like holding a player back by the jersey)
An 8-point try on the other hand is when misconduct occurs whilst an attacking player is 'touching down' the ball, and no other subsequent period. This general means the period just before and just after the ball is grounded. ( The opportunity to score the six points for a converted try and a penalty kick for the infringement)