50% off is equivalent to sometimes being normal and sometimes being 2x normal. Likewise, 90% off is equivalent to sometimes being normal and sometimes being 10 times normal.
If something costs $10 "normally" and is $1 with a "discount of 90%" you can just as easily say it costs $1 "normally" with the $10 being 10 times "normal". It's purely arbitrary which one you label as the normal price.
I have no idea what argument you're trying to make, or if this is just some bizarre semantic thing. The normal price is the non-discounted price. In some domains like clothing where there is a perpetual 50% off a price that never is set that high, that's an exception - but one that has nothing to do with Steam.
If something costs $10 "normally" and is $1 with a "discount of 90%" you can just as easily say it costs $1 "normally" with the $10 being 10 times "normal". It's purely arbitrary which one you label as the normal price.