I think expecting the soot level to be reported as 0 is unrealistic.
Consider the honeycomb of a DPF.
The DPF can become blocked due to soot which can be burnt, soot which cannot be burnt, DPF filter pieces which break off, catalytic converter pieces which come from any upstream catalytic converter, and any foreign matter which made it's way through the engine. I've seen pictures off all of the above in various makes and models of vehicles.
The pressure reporting of the DPF is done through the pre and post DPF pressure sensors.
If either of these is not exactly the same as the other, then a pressure differential may be reported which does not really exist
All of these factors means that the estimated soot level of a DPF filter, which is measured only indirectly, being derived from a pressure differential, may not be exactly correct, and may change over the lifespan on the DPF and vehicle.
To determine exactly why the DPF is not going to zero derived soot levels, do the following:
1. Replace both pressure sensors with known good units where you have calibrated them in a laboratory environment
2. Remove and cut open the DPF filter to determine how blocked it is, or, fit a brand new DPF filter which has been tested and measured to have zero pressure differential.
Only after having done the above can you reach any known accurate conclusions.
On a practical note, most people would not be prepared to spend the time and effort and may not have the resources to do the above, and thus may choose to live with the current DPF characteristics.
In other words, accept that imperfections exist and get on with life
