Maybe what they’re singing is more right than what’s written.
Whenever I write a new piece for an ensemble, I pay close attention to where the ensembles diverge from what’s written. From breaths and articulations to rhythms and even pitches, I take seriously what a good ensemble “hears” innately.
In some cases, it’s to be corrected in the ensemble. If they’re singing a major third on a minor chord, that’s a mistake I won’t stand for.
But often, the lines that an ensemble interpolates into my pieces are more natural and musical than what I wrote at the piano. So long as they don’t undermine the intention of the piece, I will often change them in my score.
And the same is true, honestly, of scores I didn’t write. As long as I understand the intention, I’ll let my ensemble slide when it comes to small divergences from the score. Heck, I’ll even make some myself.
Tread lightly in doing this – it’s someone else’s creative work, after all. But with the right musical intentions, the right attention to the score, and the right instincts, you can craft a piece into something that fits your ensemble better than what’s written on the page.