[Signman]

In may be over simplifying but both indicate the same thing: The amount of oxygen available in a defined volume of air.

In the 2000s O2 was seen as a good indicator but sensors required re calibrating regularly. Previously Density Altitude calculation was the ticket particularly when O2 sensor reading was included.
Many were for years looking at vapor pressure, humidity & temp to indicate available O2. In the last 10 years grains of water has become the indicator of favor since I believe does the VP/H/T calculation for you.
Am sure with some time will find one is more reliable than the other these days grains is favored.