as RogerWilco wrote - spec sheet only lists maximal consumption. That is, with high power wifi modules installed and configured for maximal power use as in - worst case scenario. Usual usage is much lower as rates with maximal power use are rarely used in working environment.
low load consumption is around 3 to 5 W (using 24V PSU) and your quoted power usage for r52n-M that 2.1W also is worst case, because using higher data rates that we are interested in, power consumption will not be as high as that.