Tuning and intonation is an interesting topic. Half a century ago the American norm for an A was considerably higher than today. In Europe and especially in Germany it was the opposite. Then over the decades something happened and the roles were reversed. Perhaps it had something to do with the quality of automobiles. Studebaker, Chrysler and Cadillac were the rulers then; now you have Mercedes, BMW, Audi and even VW (which manufactures today’s Bentley, a real 'People's Car'). Pitch goes up with the better cars and comes down with the lower quality! It is difficult to go and perform in Europe. Sure, a string instrument can be tuned up but at least someone with perfect pitch will have to spend some time adjusting. I don’t know how wind and brass players do the transformation; perhaps they have to use different instruments.
So, we are officially at A=440 on this continent. Some of us are at least. Since 1960’s well known string teachers at Juilliard were busy telling their students: “Dearie, you have to tune sharp to be heard.” I guess this was intended for soloists, but many took this to mean they should tune above the official A at all times. Then enters the violin soloist whose instrument is tuned easily a quarter tone sharp. Accompaniments are one of the few moments when an orchestra string player can actually hear oneself and, as a result, tries to match the pitch of the soloist. The rest of the orchestra obviously cannot follow, and often overall intonation during violin concertos is absolutely terrible.
The ear is a funny thing. Indeed, a slightly sharp note is much better tolerated than a flat one. By tuning sharp, if the soloist happens to play flat, it will still sound acceptable because the pitch might actually have fallen to the correct level. Since these soloists often get rave reviews from ignorant critics, praising their intonation, just about every other violinist wants to emulate them.
Hearing music played absolutely in tune can be almost intoxicating. Ear training should be far more extensive and every music student should learn to understand the complicated ratios all intervals have. How many average musicians know that every whole and half step in the Western scale we use is slightly different? Perhaps it is a hard concept for a pianist or other fixed pitch instrument player to comprehend, but a revelation to string players, and yes, even singers. Teach the math behind music, it is fascinating.