Have you read the Walther recall information page? It addresses two of your three questions. As to "why some and not others", I don't believe anyone knows, specifically, at least for now.
"Walther Arms, Inc. has recently discovered a potential safety issue with certain PPS M2 pistols. Walther is voluntarily initiating a recall to protect the safety of its customers because under certain conditions it is possible that some of these pistols may fire when dropped."
The recall was announced four days ago (Nov 18th), so nobody has had the recall completed. Per Walther, the lead time is 6-8 weeks from the date of receipt.