They can both scale off the same feat, that feat acts as a benchmark for the scaling. But that doesn't change the fact he scales higher.
It's like, take DBZ for example, both end of Namek saga Vegeta and Final Form Frieza, are both scaling off Frieza blowing up planet vegeta, that value is the same. But that doesn't mean they're the same strength, we know Frieza can one shot Vegeta, straight up kill him, literally did.
Armstrong mutilates Mistral. They might both be scaling off the same feat, but yeah, obviously, there needs to be a feat to get them where they are to begin with, doesn't mean there isn't a gap.
Ignoring Mike Tyson could kill a 10-A in one hit to the head, and ignoring how a 10-B can kill a 10-B pretty easily, that's what upscaling is for. It's why the "at least" is there.
No, Mistral would be downscaling the value, while Armstrong is like two one-shots above the value.
If for example, someone makes a match of Dark Blue Moon (7 tons) vs an 8 ton dude, they'd be pretty even. But if someone made Peak Star Platinum vs an 8 ton dude, that 8 ton dude is omegafucked. Even though Plat is scaling off the same feat, he literally one shots dudes on that scale, pulverizing them, even though the feat used as the foundation is the same, that doesn't mean we don't factor in the nuances of things such as Plat dusting dudes on that level.
Profiles can have nuance, dudes of the same tier can be stronger than each other by a lot, dudes can be scaling off the same feat but by differing amounts, etc.