I measured the distance from the left to the very border of the stripe on both bills.
I also took 2 measures on each bill, 1 on top & 1 on the bottom:
#...259
left to stripe -top: 18mm
left to stripe -bottom: 18mm
#...648
left to stripe -top: 23mm
left to stripe -bottom: 23mm
So there's no angle, but a precise 5mm distance between both strips. Maybe the "angle illusion" came from the fact my bills weren't lined up properly
.
I wouldn't say that this is an error, though they're the fartest strips I've find by now, which still makes them interesting to me.
Thanks to you all for your comments