Well, it’s very easy.
Let’s take a look on the sanskrit word yog(a) in the original script, Devanagri (pronounced Devnagri).
This are two letters.
If it would be Yoga, than the second part of the word would look like that.
So the correct and original word is Yog. While I’m traveling and living in India I met a lot of people who told me, that it is actually yog. So I felt like to use the right ancient word.
Actually there are a lot of words, where kind of the same mistake was taken. Examples for that are Pranayam(a), Bandh(a), Moksh(a) or as we see in the second sentence of this post the word Dev(a)nagri.