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.