General Managerial Accounting question:

So on this Excel project im doing, it asks for the Contribution margin ratio. When I look up on how to calculate it, it describes the calculation as (revenue-variable costs)/revenue. Nothing worked until I came on here and found someone calculated it as: CM per unit/ selling price per unit.

The second calculation worked but as I am doing research i cant find this explination on this calculation ANYWHERE. Can someone exlplain this to me? im confused on why this correct calculation isnt shown anywhere.