Why are ironing instructions on cotton shirts inconsistent?
Another possibility is the thread. 100% cotton is the fabric, but thread is usually a blend. The blends sometimes don’t take heat very well and either melt or deform radically when exposed to heat. Other possibilities are that the weave of the thread Note: I know nofink about textiles; I just use a lot of “reclaimed” fabric to sew things for friends (i.e. dog beds out of a dog owner’s old flannel shirt) and repair a lot of my own clothing, and I’ve made some amazing screw-ups.