If I understand it correctly, and it is perfectly possible I don't, Ancestry is pre programmed, so to speak with a large number of locations that it knows, and particularly for the US will fill in the correct county for you as well. so if you pick one of those pre entered drop down options, it works perfectly. So far so good.
The problem is, it adds every location you type in, as well as any location that is read directly from a census (even mistranscriptions) to that drop down menu. Sadly, even if a location is added manually by you entirely accurately, Ancestry doesn't recognise that name, so will reset it to the closest match it can find in it's known list - in this case Tiverton, Cheshire, which it "knows" rather than Tiverton, Devon, which it doesn't.
It seems to me it would be much better if the program didn't keep adding to it's drop down menus, but only included the place names it actually recognises - after all, how can the user know the difference between "Tiverton, Devon, England" that is on the list because they typed it in full on a previous entry, and "Tiverton, Devon, England" which Ancestry knows and recognises?
My own personal favourite is when I carefully typed "Wishaw, Lanark, Scotland" and Ancestry changed it to "Wishaw, Lanark, Scotland, Texas, USA"