Why don't they understand that the SLOC hasn't changed, but the software does more than it did yesterday because you've added new features, or fix bugs?
Now explain it to them like this. Measuring how much work was done in your code by comparing the lines of code is the same as measuring how many features are in your cell phone comparing it by size. Cell phones have decreased in size over 20 years time while adding more features because of technological improvements and techniques. Good code follows this same principal as we can express the same logic in fewer and fewer lines of code, making it faster to run, easier to maintain, and simpler to understand as we improve our understanding of the problem and introduce new techniques for development.
I would get them to focus on the business value returned through feature development, maintenance, and bug fixes. If whoever is happy with the software says they can see improvement don't sweat the SLOC.
Go read this:
https://stackoverflow.com/questions/3800707/what-is-negative-code
