Many critics include some sort of grade for the movie, album, TV show, or game they are reviewing, whether it is on a 5-star scale, a 100-point scale, a letter grade, or other mark. However, plenty of other reviewers choose not to do this. Hey, that's great... they want you to actually read their review rather than just glance at a number. (Personally, we at Metacritic like to read reviews, which is one of the reasons we include a link to every full review on our site....we want you to read them too!)
However, this does pose a problem for our METASCORE computations, which are based on numbers, not qualitative concepts like art and emotions. (If only all of life were like that!) Thus, our staff must assign a numeric score, from 0-100, to each review that is not already scored by the critic. Naturally, there is some discretion involved here, and there will be times when you disagree with the score we assigned. However, our staffers have read a lot of reviews--and we mean a lot--and thus through experience are able to maintain consistency both from film to film and from reviewer to reviewer. When you read over 200 reviews from Manohla Dargis, you begin to develop a decent idea about when she's indicating a 90 and when she's indicating an 80.
Note, however, that our staff will not attempt to assign super-exact scores like 87 or 43, as doing so would be impossible. Typically, we will work in increments of 10 (so a good review will get a 60, 70, 80, 90, or 100), although in some instances we may also fall halfway in-between (such as a 75).