This is very true. for my part our observations date from a time when NO men and very few women dyed / bleached their hair. Of course, migrants move inland, so spreading the gene (and dilutiing it too). Something to remember is that these things all took place since the last ice-age meltdown (circa 8-6000bc).I'm curious if these figures could be misconstrued from people hair dying? In iberia at least many women dye their hair blonde and ginger... I had many in my family who did it. Not super uncommon in more northern areas either. Not discounting the fact that red hairs naturally exist btw, just questioning the accuracy. Personally it gotta be extremely rare even in the northern fringes of iberia to find naturally ginger people. More common in more endogamous northern european fringes, you can find it in Russia as well. So not just on coastal areas.
There is also a real possibility that a group of EASTERN European/Asian people who were hunters & reindeer herders, came West in about 8000bc, moved north as the melt took place and finally left around 6000bc after they could no longer get to America on the Arctic ice-flow bridge which stretched from Northern England/Southern Scotland via Iceland & Greenland to S.East Canada/N.East USA. It was they who built the stone henges of Europe. When the the bridge melted a7-6000bc they simply left and the beaker people moved in! It is possible they were looking to circumnavigate the globe ON FOOT, we know that they had already gone EAST into N. America via the same type of ice-flow bridge about this time!