The theory is that the inharmonic tones that occur when you strike a glockenspiel dissipate quickly into a single pure tone, so they don't make any effort to tune the overtones when they make the bars.

Something sort of like that happens when you hit a guitar string--the tone that sounds when to strike it is off-pitch, and quickly narrows in on the pitch as the sound decays.

One has to keep in mind that human beings have had electronic tuning devices for only a short period of time, and, prior to that, "standardization" was much more subjective--add to that the fact that tempered instruments are all, in their own way, out-of-tune by design, and it's a wonder that anyone has ever managed to play music at all.