Have gone almost full circle and talked to developer and builder as well as gathered info from great people here on the forum and lots of other people. I have come to one conclusion, that being that my house and my neighbours houses were built according to the plot plans. (with one exception)
What I don't understand is why they were designed that way (6" ) difference. Builder says that is a question for the developer. Asked developer for a subdivision plan, got a totally random comment in return (email).
I also don't understand how my backyard can physically get to the grade that it is supposed to be. It looks physically impossible to top it up. Will discuss with builder when we meet.
Getting back to the one exception, my plot plan and final grade cert are very different in my opinion. Especially the swale in the backyard. I believe the elevations are correct, but the grades do not seem to reflect what is really going on in my yard.
I will try to post the plot plan and final grade if anyone is interested in comparing the two.
Thanks everyone.