I don't understand the 5 minutes part, wouldn't getting to 1st place within 5 minutes make it obvious you're cheating?
I've won with only 5 miutes left before.
Groups in FM are sorted by when you log into the game. For example, people who log in at the start get sorted with each other. People who log in 14 hours affter it starts get sorted with others who log in at a similar time etc...
What tends to happen is that people who are "tryharding" the event (trying to win, also the most likely to report people for cheating) tend to get on as early as possible to get as high a score as possible. Legit players who aren't really bothered and compete casually log on normally as well, meaning close to the start of the event.
By logging in with 5 mins left, you get sorted with other latecomers, most of whom don't even know that the event is on or are new players. You can win the reward not by getting a really high score, but by being the only player who has actually got any points in your group. I think I won at one point with only 200 points.