Why is this film so underrated?

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Hey everyone I recently brought a dvd second hand for 75p from CEX (computer exchange). I watched it, and i thought it was truely brilliant. It was a film called Ballistic, Ecks vs Sever. Its got all sorts of gun fights, explosions, and so on all the way through. And yet, when i took this film to a movie night, noeone had heard of it. Granted, i never heard of it before i brought it, but i normally don't know the name of many films. What is more, is that they decided to look up on a film review site how good the film was, and it got a measly 2/10! I dont understand this. And then there are films like the Harry Potter films that get around 5 stars out of 5, and they are just terrible! Can someone please explain the system that these sites use to review films, becuase i am totally lost.