The official tennis rankings are bobbins. They're OK at doing what they're designed to do, which is to force players to play lots of minor tournaments, but they aren't a good guide to how good players are or which other players they can beat, particularly at major events. This isn't necessarily the fault of the ATP and WTA - it's the old story that when a measure becomes a target, it loses its value as a measure. Arguably no official ranking could ever be a satisfactory measure.
The Grand Slam Tennis Ranking attempts to provide an alternative. We believe that performance at grand slam tournaments is the best guide to performance at grand slam tournaments, as well as being the only reasonable measure of greatness in a sport where public interest is so focused on the big events. Also, unlike the official rankings: