Not the king of Scotland: shoe size on Dunadd

We visited the hill fort at Dunadd where they made the kings of Scotland when the first settlers from Ireland arrived 1500 years ago. The idea was that the would-be king put a foot in the imprint in the rock. The notice said it is size 6. They must have been smaller back then. I use show size as an example of discrete variation. If we measured people's feet and found foot lengths of 13cm and 14cm, we could easily find people of 13.1cm, 13.2cm etc. This is continuous variation. But between size 5 and size 6 there is only size 5.5. They don't make size 5.28. That's what makes it discrete.