I am comparing apples to apples, you misunderstood what I am saying.
Take a fired cartridge and measure it with the Hornady gauge, then measure it with the Sinclair gauge. Now you have the fired measurement of that case with both gauges.
Now bump the shoulder back .005" using the die setting from the Sinclair gauge.
Now measure the headspace using the Hornady gauge, it reads you only bumped the shoulder back .001" compared to the measurement you took using the Hornady gauge to begin with. So according to this gauge and measurement you need to bump the shoulder back .004" more to reach a total of .005" headspace using only the Hornady gauge.
But you take that same case that is reading the shoulder bumped back .001" and measure it with the Sinclair gauge and it reads that the headspace (shoulder bumped back) is .005".
So if you ONLY had the Hornady gauge you would think you only bumped the shoulder back .001" and would set your die down more to get the other .004" needed to get the .005" total shoulder bump, when in fact you have already bumped the shoulder back .005" according to the Sinclair gauge.
So how is this comparing apples to oranges, both measurements were taken with both gauges before and after sizing and according to the Sinclair gauge the shoulder(since that is what it's really measureing) has been bumped back the .005" and the Hornady reads that it needs .004" more.
Both measurements were taken with the same brass using both gauges before and after, one says it's good and the other says it's not.
So how does one gauge say it's bumped back .005" and the other say it's only .001" yet you say both gauges read the same and are correct?