I would take that with a huge pinch of salt. Census data is self-reported, and most people don't know that Scots is a distinct language from Scottish English, albeit one sharing the same Anglic root and with a lot of borrowed words and phrases.

I live in Aberdeen, and I'm pretty sure if I was to leave my flat and ask people I met in the street whether they spoke Scots or English, I'd get a disproportionate number of Scottish English speakers tell me they speak Scots; I'd also probably get some of the Doric-speaking locals tell me they speak English, even though Doric is actually a descendant of Scots.