Also, it could have been a very loving and stable environment. You probably cannot ever know this.
True, we can only speculate. We have grandparents here who are parenting their grandchildren whose parent is BPD, and from the few stories I've seen, seem to be very dedicated and loving. Who knows how your exBPDgf reacted to her parents' neglect, Willingtolearn?
I also wouldn't discount that development of BPD has a large part to do with inborn traits like emotional resilience. The lack thereof, often in combination with neglect, abuse, or growing up in an invalidating household, can contribute greatly to the development of BPD. On the other hand, we also have parents here who have multiple children who don't exhibit BPD traits, except for one of their children.