Keith,
I'll start by explaining how string pictures are used consciously.
The string picture is the blurry vision of the string at full draw. It can be anywhere in relation to the riser and arrow (there is no generally correct place for it to be). However, if you are shooting consistently, it will always be in the same place. This indicates uniform positioning of the arrow in relation to the head and eyes from shot to shot.
If you are using the (whole) arrow alone for horizontal alignment, you can ignore the string picture. It will still be in a consistent place, but you don't need to register it.
However, some archers prefer to ignore the shaft, rather than the string. Instead of aiming with the full shaft and tip, they take care to align the string properly (actually using that consistent string picture as a tool) and only aim with the tip of the arrow. Both methods can be consistent: aligning with the shaft, or with the string picture and tip. Obviously, sight archers almost always use the string picture, because they can't easily use the shaft. But barebow archers have a choice - if they know of both methods. In reality, many don't even know that the string picture can be utilised.
People who use the string picture on purpose usually tune their bows to have it in the same place on every bow. For example, they will tune each bow for the string to be flush with the inside of the riser in their full-draw vision of things. But once again: there is no 'correct' string picture. It's personal - and can just as easily be right next to the arrow, for example. You choose a place you like and tune the bow that way.
However, some people don't fully realise that the string picture is part of their overall sight picture. They actually use it as a reference, but are cognitively unaware of it, or rather: They only register it as part of 'the big' picture of the bow, arrow and target... as part of 'everything looking okay', as it were. This being the case, it doesn't always feature systematically in their tuning efforts. And therefore, they may employ a different string picture on each bow. For some reason, the shape and look of a given riser moves them to tune it in a certain way - but they'll tune the next bow differently. Or two the same, but one differently... and so on.
You can easily see what might happen. You can have a bow you shoot very well, then tune two or three other bows to a different string picture for some reason or other... shoot them for a while, then move back to the original bow. And all of a sudden, the confounded thing does nothing but confuse you. You force alignment into a string picture that is wrong - for that particular bow - because you've got used to that picture with your other bows. Suddenly, your favourite bow no longer makes sense to you.
All this is speculation in your case, because many archers don't use the string picture at all. But if you are one of those who uses it, but doesn't fully realise the fact, it's an avenue worth exploring.
Best,
Martin
Edit: Next time you shoot, simply take a good look at the string picture and try to ascertain whether it means anything to you. If it feels important, you're very probably using it.