Not to detract from the topic, but...
As long as the loop at the end is done well isn't a string a string?
No--not at all. Bundle tension, tapers, back twist, serving, etc. all make a difference.
One wouldn't stop stretching, one was too long, despite me sending measurements & 2 from the same maker broke before they'd got to 20 shots!
With the stretching, unless a loop was slipping it was the material. Some materials, especially dacron, keep on stretching. Low strand count strings tend to stretch more also.
The one that was too long...could have been a goof, could have been two different ways of measureing. I go by AMO (measure under tension), but unless the string is measured under tension it won't be the same.
The two that broke...I hope you contacted the string maker. Sounds like he/she had a spool of faulty string material. I've had that happen to me once or twice over the years--no way to tell until the string fails. It's rare, but it happens. The only other thing I can think of is a rough spot in the string groove, if they broke in the loop.
Chad