English tree widths are measured from the tip of one tree-point to the other. Since different manufacturers use different lengths on their tree-points, this measurement doesn't mean much unless you're familiar with each specific manufacturer.
Geometry: Here's a tracing of my Passier Medium width tree (as close as I could get my flex-curve to follow the tree)
Let's say you have a saddle with tree points that come down 15 cm from the top of the gullet instead of the 11.5 cm that I have. If that saddle still has a 27.5" tree width, then obviously the angle of the tree is going to be much narrower than mine. So when you have a measurement for tree width in cm, it means absolutely nothing unless you know the shape and geometry of the tree itself.
In general, a medium English tree is going to have about a 90° angle, while a wider tree has a larger angle and narrower has a smaller angle.
Not only the tree angle, but the length of the tree points will affect the fit at the front of the saddle. And also remember the the profile of the saddle (along the length), which is the tree shape, not width, also affects the fit.