It doesn't have to.
I need six hours of sleep. Four hours in, my sleep is interrupted for twenty minutes by a random encounter, hauling the dead wolves out of the campsite, cleaning my sword and bedding back down. I still need two more hours of sleep.
I think the operative points are:
• The long rest must be
at least 8 hours.
• The sleep within that long rest must be
at least 6 hours.
• The light activity can be
no more than 2 hours.
• Any adventuring activity that does not interrupt the rest and require a restart will be
less than 1 hour.
There is no scenario in which
less than 1 hour of non-interrupting adventuring activity would require the long rest to take more than 8 hours. It
might interrupt the 6 hours of sleep, which
might either cut into the
no more than 2 hours of light activity, or the PCs may
choose to extend the long rest. But that’s up to them.
Also of interest: The PCs could, of course, do
more than 2 hours of light activity, but the excess won’t be part of the long rest. If this occurs in the middle of the rest (rather than the beginning or the end), this
might mean that it has to count as time spent engaged in adventuring activity.
Unless you interpret walking as the only adventuring activity that might not interrupt, in which case, I suppose
any light activity in excess of 2 hours that occurs in the middle of the long rest forces a restart?