There are three separate and unrelated issues being discussed here.
The
first is the Intonation issue Landru mentioned, which is why guitars never play scientifically "in tune." I've always liked Kevin Ryan's overview and Tempered Tuning method
http://www.ryanguitars.com/NewsandEv...ning_Terms.htm, but there is also an entire Google-search world full of other good explanations.
The
second is the "going out of tune" and "metallic ping" sound issue, which may be caused by several factors, including string balls not snug against the plate, a loose tuner, or an improper nut slot - which would account for both.
The
third is the overall "sound" of the string. That one's a head-scratcher for me.