Ah, so you mean the ability description. Keep in mind, the effects of the buffs are additive and going to apply to the same thing, but those effect descriptions are auto-generated and don't always give the most accurate picture. It's looking at the buffs individually. The key thing is the newer buffs don't replace the other buff. So, when you drop to 50% health, you're getting a buff that takes away 50% of your damage, and when you drop to 75% health, you're getting another buff that removes 25% damage, but they're both applying at the same time, so you're at -75% still. If you look at the debuff section of the unit card when the unit is damaged, that takes the sum of all similar debuffs and displays them as one number, so you can see it working properly there.
The ability also says it's applying a 75% chance to dodge on NR ships, but in reality that's applying to the ship's fighters, not the ship itself. There's a way we could make the health/damage amounts display correctly independently, but there's more possibility for edge cases where it's not removed properly as the ship heals that way, so I've always been reluctant to do it.