Not accurate

I have a slower swing speed but I know my distances and it is not accurate. No matter how well I hit it, it is always 15 or 20 yards short of what it should be. Shorter clubs seem to be accurate but once I get into the 9 iron and up it does not work.

I carry my 3 wood 20 yards further than the roll out it's telling me. Not impressed. Maybe only works for guys with high speeds???