How does the scroll system work?

There's some problems with the RNG. I had over 90% success rate across samples larger than 100 with 70% potential scrolls when I scrolled as fast as I could using Legendary Spirit. This happened on 3/3 samples and never produced items above Rare. I was potting Phantom cards after Phantom release.

Sadly all that can really definitively prove is that you had fairly decent luck/results when doing those. Without being shown the formula they use to generate these numbers, any set of data values, no matter how many there are, cannot, beyond a reasonable doubt, prove anything. You can make assumptions as to if their system stays true to the claimed percentages or not, but that is about all you can do.