Hi - There are several reasons why GPU mfgs suggest higher wattage PSU's than are actually needed:
1.) To allow for those PSU brands/models that lie and exaggerate their power specs, and a lot of em do it.
2.) To allow for older PSU design's that simply don't allocate enough of their total power to the +12v rail(s)
3.) A recognition that these PSU's are not only used for gaming/gen purpose but also potentially for
bitcoin/litecoin mining which is far more taxing on the PSU.