There are dozens of different ways to evaluate the "performace" of something, and it is generally much more difficult to translate a marking scheme that has less precision to one that has more precision ... For example, a pass-fail system is heavily used in various forms of certification and it would be (nearly) impossible to assign an appropriate percentage grade when you only know that someone passed or failed.
The problem with Metacritic is they assume that every grade on a scale represents an equal distance from any other grade and therefore a pass-fail represents 100% and 0% respectively and the letter grading system is (roughly) as follows:
F-: 6.66%
F: 13.33%
F+: 20%
D- : 26.66%
D: 33.33%
D+: 40%
C-: 46.66%
C: 53.33%
C+: 60%
B-: 66.66
B: 73.33%
B+: 80%
A-: 86.66%
A: 93.33%
A+: 100%
The truth is that most people are used to letter grades in the educational sense, and the breakdown is much more like:
F: <45%
D: 45% to 60%
C: 60% to 75%
B: 75% to 85%
A: 85% to 100%
Now, the (probably) haven't received too many complaints because at the high end their scale isn't (that) much different from what people would expect and most games/movies that people are opinionated about tend to be close to the higher end of the scale.
