The decimator has an annoying habit of generating flipped faces in the result [particularly on flat surfaces], and zero area faces [also often on flat surfaces]

somewhat related [mostly so you can see my instructions give different results for you], it seems to give different results in different builds/versions

to see what I mean, subdivide a plane 3 times, then decimate to 115 triangles

I've attached a pic of the problems, zero area faces circled in yellow, flipped face circled in blue