Stretch indeed!
There's abit of give in all sorts... if you release the luff line and stretch the sail.. you'll be surprised how far it will go.
Its more in the knots that hold all the bits together.. halyard, pole bow ropes...it all compress under load. We like to have the luff tight if a nice deep entry and then flatten the entry as required by easing some halyard - works good and took us a few solid races to get the luff length right...