The minimum PSU rating assumes a normal sized computer that has several drive bays (not just 1) , several HDDs, and several expansion slots with devices, and 4 memroy slots that this computer doesn't have. Dell under rates their PSUs because when you're selling computers to companies by the thousands efficiency adds up to a lot of money.
$20/Year X 1000 computers X 5 years=$100,000 So does reliability when they're all on service contract and extended warrantee. dell PSUs are probably some of the best in the business. certainly above average.
In the MT case Dell offers a 305W PSU, and a 255W EPA PSU. They both have the same output on all power rails. the 255w is 85% efficient, the 305w is 75%. So Dell gives the EPA a lower rating to show it's efficiency . Dell doesn't sell PSUs by the Watt like the aftermarket "Advertised" Watts you're used to. people have been running GTX750s on that PSU for years.
Actually if I were building that thing it would Have big PSU wired out the back as a power brick and an i5-2500k overclocked with Throttlestop software. But that's just me.
The 1050 draws 75W the 750 draws 65W The PSU has enough power and then some.