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My assumption of that is you have an outbreak that is extremely severe, the medication can be used up to that level and still be "safe" for the fish. If you look at the thread link above it might make a little more sense.
2.5ppm chelated copper could be needed to tackle a copper resistant strain of ich or velvet. For example, if a fish has been continuously exposed to subtherapeutic copper at a LFS or wholesaler then in theory the parasites could start becoming resistant to whatever type of copper was used.
Another reason they might recommend 2.5ppm is to ensure the copper level remains therapeutic; knowing many end users will use rock/sand in their QT, won't test, etc.