Yes, it is quite normal.
I assume you havent noticed if it was longer from the start ?
Jumping on one side only might be the cause in theory, but I honestly doubt it is the reason.
Most likely just standard stress on the lines, that WILL cause some (even prestretched) lines to become different in length.
Just move one of the rearlines on the leaderline knots, so equal length rearline ends seen from the bar, OR, move on knots till your kite flies straight when overhead, both when sheeted fully out, and when sheeted fully in (especially the last part, as it is the only part that concerns the rear lines).
Why do I write the last part ?
Because, each and every kite will have a slightly but not noticeable offset, and many has a noticeable offset, so you might have to make one rearline a bit longer than the other, so the kite flies perfectly straight.
I am NUTS regarding kites that dont fly perfect, so mine has to be trimmed bullseye, otherwise I get more annoyed than even seaweed in large numbers
(and yes, I AM difficult, I know...)
If the rear lines (AND front lines of course) are equeal lenght and still it flies a bit to one side, then if you use the same bar for several kites, you should change/move the knot on the rear line pigtails at the kite instead of course
Peter