Approved I485 for onely one of my parents why??

[jxpm]

Has any one been in a situation where even tho both parents i485 have same priority date only one of them was approved and the other one is still pending. Has anyone been in this situation and have an idea how long it took for the other parent to be approved?

Thanks in advance.

[Enub4]

I have read it a couple of times. How long will vary from case to case, I think I read a case that took a few months and another a year or so.