I’ve never understood AMD's affliction to over-stating the PSU. When a 290X or a 780Ti had basically the same average and peak power, Nvidia states it as "Minimum Recommended System Power" of 600W, while AMD denotes, a PSU of 750 is recommend. It's like AMD has to shoulder the worst case scenario, like OC'n both CPU/GPU and other power sapping components, or they take the onus for someone’s crappy over-rated PSU from 2009.
It's well documented and discussed that most contemporary 550W PSU's of acceptable quality are more than satisfactory with the bulk of R9 290's. If you check Guru3D's power supply recommendation for a R9 290 DirectCU II OC Edition which has 1000Mhz boost; he's indicating for the average system a 550~600 Watt PSU is acceptable.
Interestingly an OEM as HP would value price, power, heat/thermals, within that chassis build, while maintaining strong performance/value. This might leave one to reason a R9 380 as acceptable on power/heat and priced for HP to consider it even over the 970, or on par to what Nvidia was willing to provide though not at as an attractive price.
Although the flip side, AMD couldn't best the 980's thermal/power envelope, and HP is willing take Nvidia pricing of the 980 to provide the "utmost" that chassis/PSU can maintain.