Measured soot values reading negative

I decided to do some logging on the way to work last night as I've always wondered if at any point in my commute the DPF actually got hot enough start passively regenerating. As it happens, it ended up resuming an active regen that I interupted the day before but the readings were not what I was expecting:

around 16g soot mass calculated and 0.26g soot mass measured

I thought I may have had a dead sensor but both readings came down pretty much together despite the huge zero error until the measured got to -7.9ish (calculated at around 6) The calculated then dropped to around 5.6 before the regen finally stopped.

The car has never had any DPF issues and from what I have noticed doesn't regen overly regularly so I assume active regens have been triggered from the calulated value and this discrepancy may have been an issue for a while as I've never checked it before.

Do I play with it or leave it alone? Thoughts from your good selves