I'll say I've had my fitbit charge 3 for a few years now, it rarely gets things exactly right. That said, unless I manually count my steps and log them by hand, I don't expect anything to be perfect.
The Charge 3 doesn't really have the sensors required to 100% track your steps or stairs or anything. I've had my HRM show 70-80, when an external SpO2/HRM sensor is showing closer to 140, my google fit and fitbit constantly disagree on steps, etc.
This isn't a flaw or issue with the charge 3, it's more in the design - there's only so much information the fitbit can derive from the sensors included. As I mentioned, the HRM can be wrong, the steps are based on your wrist movement, stairs are too, so there's a lot of data produced from a few sensors. It's a miracle they come as close as they do most of the time - which is a testament to fitbit's heritage and abilities.
Long story short: the tracker will be wrong, it will often be wrong, but it's probably pretty close most of the time - which is what people are looking for. Additionally, it takes time for the charge 3 (or any tracker) to adjust to the user wearing it, we all step differently, we all have a different pace and gate on our step, some may limp all the time, some may limp some of the time, your steps are frequently uneven, etc. Whether walking, or going up a flight of stairs, everything changes all the time, and with the limited amount of information the charge 3 has to go off of to determine those factors, it may take a while for it to adjust to your way of doing things, and give you more accurate output.
As fitbit packs more and more sensors into their devices, things will get more and more accurate over time.
But hey, congrats on the lighthouse achievement.
