I don't know why there is a difference in listed max output resolution. Maybe Sapphire's site has an error.
Maybe Sapphire has a different standard in how they report max resolution than Gigabyte. For example, if they listed the max digital resolution that is supported at 60 Hz without compression, that would 5120x2880, as per the limitations of Displayport 1.4 (which would be the same for the Gigabyte card).
Maybe Sapphire doesn't support compressed output, so the max resolution really is 5120x2880/60Hz.
Regardless, it's not something you have to worry about unless you actually plan on using those resolutions.