Sadly the set up on Ancestry allows you to have as many trees as you want.
Many researchers will have a 'real' Private tree which they have researched meticulously. Then a slew of Public trees for different branches of the family, on which they add all the Ancestry 'hints', so research done just a load of info added, some may re-visit these trees and move data about.
Due to people have what I call a 'hint' tree I have found some really odd things. One poor soul died so many times it was quite funny. The owner had obviously added every death found in that name!
However, there are some very good Public Trees on there. I would say that in general if the tree owner is in the US, these poor people are so anxious to find their English roots, they will add all and sundry on with gay abandon. Their excuse is that it is so hard to research in the UK if you are not living there!
But we in the UK are far more likely to have done some research.
I know I have a tree on Ancestry and I can assure you that I have researched it, spend a fortune on BMDs and research in libraries all over the country. It is Private, but if anyone can prove they are connected I am more than happy to allow them access. Many have failed my exacting standards!