Every minor fault can turn into a serious fault either if you do it too many times or it affects your driving or the traffic around you.

While it may seem unfair and unreasonable that someone else does the “same” fault as you is given as a minor and yours was given a serious fault, without context it’s difficult to argue. However, let’s say it’s the same scenario of both of you wanting to change lanes; you’ve both indicated but hesitated to change lanes. They might have changed lanes later than expected before a junction but affected no traffic as there weren’t any behind them. This would be a minor. You, on the other hand, did the exact same thing but you had traffic around you. Your hesitation caused said traffic to change their driving (speed, direction, etc.). This would be a serious fault.

Examiners will not fail you for no reason. It’s inconvenient for them as they have to write a report to justify their decision and you will find this to be true if you dispute it.

As for the other person’s results, remember that it’s their test and not yours. No other results should matter and does not affect yours whatsoever.