Basically, two possibilities:
1. If you have multiple I-140 approved BEFORE I-485 is filed, your 485 is
linked with you most RECENT I-140.
1.1 Sometimes, USCIS automatically certified your PD date of your later I-
140s to be the earliest PD. But it's not guaranteed.
2. If you have an another I-140 approved and while your I-485 is pending,
your I-485 is still linked with your earlier I-140, thus the earliest PD.
There might be more possibilities.