Interesting observation in the case of the acidifying primo impasto

I thought this was some kind of fluke when I first saw it, but it has now happened in my last 5 or so panettone bakes, even with different pasta madres.

I let the dough bulk ferment in a Cambro container and then use a 250ml beaker with 80g of dough to keep track of the rise.  After the dough triples, I always notice a significant difference between the pH of bulk dough and that of the spy dough.  The latest is about 4.95 in the beaker and 4.45 in the Cambro.

Any idea what might be the cause of this?  I thought maybe it was related to oxygen exposure - in the beaker, only the top of the dough is exposed to oxygen whereas in the Cambro, the entire surface of the dough is exposed to oxygen except the bottom which is touching the container (dough is so small compared to the container that it doesn't touch the sides of the container until it has nearly tripled).

The cambro is covered with the lid and the beaker with cling film.  Both are left to rise in the same conditions, right next to one another.

I can think of some ways to test this, but thought I'd ask here as well since testing will take some time.