Diving - Issue with NDL canculation when ascending

Maybe different algorithms or different settings for decompression conservatism?

I'm not sure what sort of "solution" you're looking for here. Garmin dive computers use the Bühlmann ZHL-16c algorithm. Some Suunto dive computers use the same algorithm, so if your buddy has one of those and you both adjust your gradient factors to match then you should get similar results. Other Suunto dive computers use different algorithms such as proprietary modified forms of RGBM. Those are always going to generate different NDLs and deco stop times.

As to which algorithm is a more accurate model of real diver physiology, that is still a matter of some debate. If you're going to push close to the NDL then you ought to learn decompression theory and don't rely too much on what any particular dive computer tells you.

Garmin devices correctly implement the documented algorithm and produce results consistent with other devices that implement the same algorithm with the same GF settings. Like if you run Garmin and Shearwater dive computers side-by-side then the outputs will be nearly identical. It is unlikely that your device needs fixing.

If you want a longer NDL then you can increase the High GF %. Or switch to a different manufacturer's device with a more "liberal" algorithm.

I'm not trying to be a jerk here but the fact that you're asking such questions indicates that you're unclear on the basics and are running risks you don't even understand. I recommend you take a step back and complete some more advanced training before you hurt yourself.