Imo the way the percentage of completion for an achievement in the watchlist is buggy or at least implemented very strange.
If you activate a multi tier achievement for tracking the percentage of completion that is shown is only that of the current tier not of the complete achievement.
Now this is pretty crazy as the only thing a completed tier gives you is some AP which are hardly worth it anyway (and could often be gained a lot easier by completing a current living story achievement or some cheap dailies) and are nothing compared to the rewards you can get for completing some of the multi tier things.
However the real problem (and bug) imo is that the tracking for monthlies that is done automatically as long as nothing is tracked manually does not work this way. In this tracking the percentage shown is always relative to the full tier not the currently active one.
This means that if you got a monthly tracked automatically and then activate the same achievement manually the percentage changes which is inconsistent.
I cannot really imagine this is by design and would recommend doing it as it is done for the monthly for all achievements.
Tracking tier progress is kinda pointless (in some cases as with the seasonal sentries vs dolyaks where the final goal was the same but the sub tiers different it even lead to a false sense of which one has the higher progress…) so I would advise a change to total achievement completion for all tracking (or a setting so the user can decide)