The choral and orchestra musicians compensate for the piano 3rds and 6ths.
As the piano plays 4ths and 3rds the 3rds have 7 vibrations a second and the 4ths are (almost) open.
This is called stretching the otaves, but in the lower end.
The same thing is done regularly for the high end to make the top notes sound correct, even though they are a bit sharp. It has something to do with the short strings being hit with a hammer and not bowed like a violin.
When a piano play a arpegio and ends on a high note it should sound in tune when in actuality is is a bit sharp.