This is all because the light meter always give us reading for middle grey.
Point the meter at black, it thinks "Humm it is dark, I need more light to make the shot looks grey." Hence, the need to dial in -EV to compansate and make black - black.
Similiar token, point the meter at white, meter think less light to make it grey. Hence, the need to dial in +EV to compansate and make white - white.
There is really no fix rule on snow should be +2EV or +0.5EV. It all depends on the scene and how the photog wants to present it as.
For example, snow with overhead sun will be flat and just 'white', so +2.5 EV probably (white with no detail). OTOH, side lid snow may contain a lot of very tiny shadows (texture!) which could very well be middle grey (1/2 Black + 1/2 White = Grey) and should shoot as is with no compensation.
Remember photo class teacher told me the rule of: "If you want white with details, +2EV. -2EV for black"... I did not truely understand it untill I came across a "
Zone System" book in a recent estate sale. And excellent explanation in
John Shawn's "Nature Photography Field Guide". Highly recommended (forewarn, it was written for film but techniques still apply)