Because an 8K pixel display is always 8K. So the display on a typical TV always has the same mode. That could mean your 4K@120 signal could (in theory) translate to 8K@60 internally. The data rate over HDMI would still be the same. HDMI is just an interface. What the TV does internally is up to the manufacturer.
Besides, it wouldn't be the first time a TV advertises a higher mode than the display can actually handle.