Google Pixel 3 software makes display corners rounder in the name of symmetry
Google itself hasn’t yet issued any kind of statement on the matter, so there is no guarantee that these changes are to do with symmetry. However, this certainly appears to be the more plausible explanation.
Oddly enough, despite the more noticeable lack of symmetry on the larger Pixel 3 XL’s display, these software modifications don’t appear to be present on the device. However, considering Google’s dedication to Material Design, the company could easily introduce them in a future update. Perhaps alongside the ability to hide the notch without deactivating the display area on either side of it entirely.