It is an incredibly important distinction to make. 2 out of 10 means every 10 seconds we must stand still for 2 or we lose DPS. 4 out of every 20 doesn’t mean that. It means in a 20 second window we can pick any 4 seconds to stand still without loss of DPS.
Try it out if you don’t believe me. Stand in front of a target dummy and force yourself to cast it exactly on CD (impossible, but try). You will find you have to wait to cast things with awkward seconds, or half seconds of waiting. Then try it normally. You will see you can fit everything else in perfectly fine.
This has an even more profound impact on movement, where we can move around for 16s without loss of DPS as opposed to having only 8s of freedom the other way.
The point is though, you said:
Which is an untrue statement. You only need to stand still 4 out of 20. You NEVER need to stand still 2 out of 10.