DBE

Mathematically speaking? By subtracting a negative background
Joke side, did you enable the rescaling/normalization option in the DBE process? That might be a the root cause for your observation.

I think you are taking the semantic meaning of the method "Subtraction" too literally. A gradient correction will fit the background both adding and subtracting values based on the samples and other configured parameters. An additive undesired gradient, yes, will generally be subtracted which is why the method was given this name (I think) because of its practical benefit. However, the correction of a gradient will add or subtract as necessary to make it uniform. Furthermore, if you place samples on dark regions.. DBE will attempt to add values there… and if they are very small and you have not constrained DBE..the result can be quite bright. 

In fact, as I demonstrate in my videos on DBE, you can actually change the value that is added or subtracted in an area by changing the calculate brightness value at any sample. 
Did you ever notice that DBE will sometimes make bright objects have a dark background near them? So people avoid putting samples there..but that isn't enough sometimes… so you can manually adjust the brightness of model (add or subtract) to clean this up.

If you are interested in explanations and demonstrations as I have described above… please become a member of my site at AdamBlockStudios.com . You will find it very informative.

-adam

Update: I cannot replicate it. I mean I opened the same image, did DBE again, this time the result is normal. 

I am 100% certain I didn't have normalize selected, or division, or anything like that. Because I never use those things. Maybe Pixinsight just threw a fit, sometimes it happens, especially with dynamic processes. 

Thank you for your replies.

Cheers,
D.