The base for the Scots language was the Northumbrian dialect of Anglo-Saxon which was spoekn all over northern England as well as southern Scotland hence hundreds of years ago when that language was being spoken it was pretty much the same whether north or south of the border. In Scotland it gradually took over as the language of the Royal Court etc and was influenced by Latin and French etc but I imagine ordinary folk in the Borders and Northumberland sounded similar still. I myself have had my own Borders dialect mistaken on several occassions for Geordie - by Glaswegians! No-one in the Southern Uplands would mistake me for a Geordie though. Depends on what you are used to hearing! I've had other Glaswegians mistake me for being from the islands. Some of our dialects are simply not heard that often in the media hence people not well travelled throughout the country don't always instantly recognise them.