wasting time tactics apply when the game is in play. players taking their time to take a throw, or goalie picking the ball, put it in 6 yard box and dilly dally before a kick, yes. thats a time wasting tactics. But during these events the clock are still ticking, refs dont blow whistke and stop his watch for this. Substitution would be in this category, because subs are made when ball is out of playing field (not to be confused with stopped play). Similarly, bringing the ball to the corner of the field to waste time is within this bracket, because the key thing here is "in play".

But events like fouls are stopped play, where ref blows whistle to stop the game and then arrange the walls etc etc, and play only starts when the ref blows whistle again. Similar goes to goal celebration, the game stops when the ref blows for a goal, and this is when the players go celebrate etc. The game will only resume at the centre when refs blow his whistle.

So in reply to ytd's game, the 8 minutes applies to all the time stopped in the 45 minutes. In normal situation where extra time is 3 minutes for example, the time stopped for fouls are negligible and some cases theres no foul at all, so usually the injury time ends on the clock. But in the 8 minute of ytd's game, there is a lenghty stop when Liverpool conceded the foul, which is the period ref blows for penalty, and ref blows for RVP's kick. Then there's another stop, where ref blows for RVP's goal and only restarted when the ref blows for kick-off in the centre. And again, There is a lenghty stop when Arsenal conceded the foul just outside the box, where ref blows for a stop after the foul, then allign the wall etc and only blows whistle when Suarez is ready for the kick.

So you see, from the moment Liverpool conceded the penalty, until Kuyt scored their own penalties, there is a whole lot of time when ref stopped game. These stops are the ones i mentioned being game being stopped for fouls and injuries etc.

At the end of the day, i do agree this is down to ref's disgression. I am just giving an explanation as to why generally, refs do go a bit over for extra time if there is significant game-time being stopped from within injury time.

PS: i tracked the time of the last few minutes of the extra time. from the moment RVP was fouled, the clock was 96:08 minute. The moment when ref ended the game after Kuyt's goal, it was 101:50 minute, meaning the game went on for 5 minutes 42 seconds after the 96:08 mark. By right it should only be 1 minute 52 seconds. But i also totalled up the time ref stopped play due to all the fouls and celebrations (i do this based on ref's whistle) and all in all, 4 minutes and 43 seconds of "stopped play" happened within the 5 minute 42 seconds. That means just over 1 minute of "active play" happened within that period. So if i add that 1 minute back to the 96:08 mark when it all began, theoretically the injury time is only until 97:08. Adjust my timing for mistakes or delay when i press stopwatch, THEORETICALLY ref ended the game 1 minute earlier. of course we all know, in football theres nothing theory about it, and whether what i said about the time stopped on fouls and celebration above, is still open for debate.
i know this is tl;dr, and this is not meant to be aimed at anyone, from any side ok? im just a stats freak and if you do enjoy analyzing these things as much as i do, i just thing it is an interesting stuff to look at, from both sides of the argument