Well, the one coming from the QScreen doesn't seem to be correct either. I've got the QScreen from QApplication::primaryScreen()
and see following:

QScreen devicePixelRatio: 1 QScreen logicalDotsPerInch: 72 QScreen logicalDotsPerInchX: 72 QScreen logicalDotsPerInchY: 72 QScreen physicalDotsPerInch: 113.5 QScreen physicalDotsPerInchX: 113.5 QScreen physicalDotsPerInchY: 113.5

According to my user the DPI he should have 227 DPI:
"13-inch MacBook Pro models introduced in late 2012 or later. Native resolution: 2560 x 1600 at 227 pixels per inch. Support for millions of colors."