If your terrain is with even number of verts , it'll have to split it in half. This would mean for example (in a row) 32 / 2 = 16.
Between the 16 and the 17 vert there is a face...Where would this face go
?
If your terrain is with odd number of verts , it'll split in half at that 17 vert...33 / 2 = 16 from both sides and the 17 at the middle.
I'm not shure about this
but this might be the reason why its adding a vert...to save the middle faces.