What is the theological justification for the British monarch worshipping in the Church of Scotland, rather than the Scottish Episcopal Church?

When Queen Elizabeth II is in Scotland, she worships in the (Presbyterian) Church of Scotland, rather than in the Scottish Episcopal Church, which is in full communion with the Church of England. The **political** reason for this is obvious, but how is this theologically justified? (Or alternately, does no justification exist, with politics overriding theology?) How can the Queen believe in apostolic succession and episcopal governance south of the Tweed, and in Presbyterianism north of the Tweed?