I think you are probably right, the scans are those which FreeBMD have available, even if they haven't yet transcribed them and they admit they don't have a complete set yet, but add to their stock as and when funds are available to do so.
Since there are more available than are transcribed, albeit incomplete, at least it's a little way further ahead than we were before and if you are careful and take note of the quarters/years and pages checked it will cut down on those needed to be paid for in 1837online.
I was listening to Radio 2 two days ago, shortly before 5.00pm there is a slot called "Miles Mendoza's Website of the Day" and this particular day he was promoting family history - surprise surprise!
His commented that "Ancestry.co.uk are the only site with the complete Births, Marriages and Death index between 1837 and the present day"
Shows they don't research very well doesn't it?

Mary