The true-up is your annual settle-up with the utility for energy your system didn't cover. Surprise bills almost always trace to one cause: your usage grew past your design. New EV, hot tub, home office, kids getting older, more AC in a hotter summer — the system was sized for the life you had, not the life you have.
Sometimes it's the design's fault (sized to a single bill instead of real usage data), and occasionally it's underproduction from a failed component. An audit separates the causes in about an hour: production data tells us if the system is healthy; consumption data tells us if your life outgrew it. The fix ranges from mode changes and habit shifts (free) to added panels or battery (an investment that beats paying the utility forever). What you shouldn't do is just quietly pay it every year.