Really nice buck. Air density is a little higher 1000 ft down. Was the humidity higher as well? That will increase resistance and thus your drop as well if it was.
There are an endless number of possibilities including a difference in wind at the mid point and POI from where you were set up for your shot that could figure in as well.
Out past 600yds, things just get a lot more complicated and of course there's always the issue of the shooter as well.
Where you shooting prone by chance here vs bench in practice? That can affect the POI as well.
SO many possibillities... .
Man you have a great buck there to be really proud of so I wouldnt' be at all down on myself if I was you.
Just keep good records of your practices undery varying conditions and over time it'll tighten up for you.
Again, what a great buck, congrats.