Beepster
Oh and when I was working with that accordian player I'd tune to her instead of to a tuner because of course the physical reeds in a squeezebox tend to have very slight deviations from perfect 440.
In a "2 voice tremolo" accordion the second bank of reeds that produces the tremolo is tuned a bit sharp, the exact amount depending on how "wet" the instrument is tuned. The tuning difference between the pairs of reeds also varies with the pitch of the note, the idea being to keep the rate of the tremolo constant. Which doesn't happen if the reeds are all tuned sharp by a fixed number of cents or Hz. Three voice tremolos tend to be more pitch-accurate because the flatter reed bank kind of cancels out the sharper one. Though they still "wobble".
Hohner once produced one model of melodeon/button accordion that had the tremolo reeds tuned slightly flat. For whatever reason, while humans find the slight sharpness of conventional accordion tunings quite acceptable against other instruments, the "flat tremolo" just sounded wrong. Really wrong.
Temperature and the amount of use/wear on the reeds can make a difference as well. To complicate things further the biggest producer of reeds and instruments is Italy, where a standard of A=442.4Hz can still sometimes be found.
Then you have the Cajun accordian tuning which is intended to make a diatonic instrument in the key of C sound better when played in C and G in a similar way to how blues harmonica "just" and "compromised" tunings work.
You can go mad trying to find the theoretically "correct" pitch to tune against when playing alongside some fixed pitch instruments. The best solution is to stop aiming for mathematical perfection and settle for things sounding good instead.