Variables in DAX

Hi,

I'm looking at this page Guided Learning , it mentions: 

"Keep in mind that each time you reference a variable in your DAX expression, Power BI must recalculate its value according to your definition. For this reason, it's good practice to avoid repeating variables in your function."

But in the video, the professor says the variables will be only calculated once, that's the reason to use it in the first place.

Am I missing something here?