I haven’t heard that before but I don’t work on hair care products very often. Benzyl alcohol is used as a preservative in lots of cosmetic products though. It can be considered an allergen for some people, but overall it’s pretty safe (as far as we know so far).
It looks like the 91% natural ingredients version has benzyl alcohol as a preservative which is typically synthetically derived and in my experience can drastically shift the bio-based ratio.
As far as I can see, the rest of the ingredients are the same, but the sourcing of those ingredients could be different which could also shift the naturally derived percentage.
Can see the before and after images in this article
It’s $3 per month for 200GB for me currently in the US.