League-of-legends – the servers

Bonus health means health gained from items / runes / masteries.

At level 18, Shen naturally has 1958 health*, so Ki strike bonus damage is equal to (Total Health - 1958)*.08.

*Shen suffers from a reduction in health due to the Inverse Ninja Law when Akali, Kennen, or both are on his team. This will reduce the base health of all involved by 1 or 2, giving Shen a range of base health from 1956-1958.

League of Legends uses a mathematical system to match up players of similar skill in the “Normal” and “Ranked” game types. Source

So just like in "Ranked," "Normal" uses its own Elo for matchmaking. It's just hidden.

Most likely, though, because your colleagues are new, their opponents aren't as good as the ones you're facing now. In other words, your colleagues' opponents don't capitalize on mistakes as much as your opponents might, because you're a better player, and the people you play against scale accordingly.

That's why your colleagues can get scores like 0/17 and still win a game.