Before going into the heart of the matter, it’s important to clarify a topic apparently obvious.
Let’s imagine preparing a Heliox 50 starting from an empty cylinder.

What does Heliox 50% mean ? 

It seems provocative but instead it’s important to stress this point: half Helium and half Oxygen.

Obvious, right ? Only apparently, half and half in what terms?

For example, 100 bar Oxygen and 100 bar Helium are half and half, but this is not a Heliox 50. 

If you add 100 bar Oxygen and then Helium to 200 bar in sequence at 20 °C  to an empty cylinder, you get a Heliox 57.1
Reversing the order of the gases, Helium first, then Oxygen, you get instead  a Heliox 47.6
And even 1,000 grams of Helium and 1,000 grams of Oxygen are half and half, but even  this is not a Heliox 50.
The misunderstanding here is even bigger: the relevant difference between the atomic weights of Helium and Oxygen causes a Heliox 11.1 !