It might be the canopy that stretches slightly.
Could give it a slightly deeper profile yes, but even worse, is if the "line" in the trailing edge does not stretch the same, then you get a concave abrupt trailing edge, which at first feel seems to give a bit more power, but also a LOT more drag (and not much depower left), so not really desired IMO
Of course, most kites (in fact ALL kites/bars I've had or seen) will have their frontlines stretch over the years, just a tad.
We all know that, so the depower is pulled a bit more every year (or we change the rear line knots)
No lines are prestretched so much, that they will not stretch when used.
And even after the first half year of initial use, they will stretch further, is my experience
But you have measured your lines when the kite was new, and now, it seems, so apparently not your issue...
It is very common and wellknown though, the linestretch over the years.
Canopy stretch, giving the kite a "saggy" profile, is my best bet, if you are sure your trim hasnt changed one inch