If you read the claims in Apples rubber banding patent, they specifically include "displaying an area beyond the edge of the document" (in iOS, the gray linen background) when you over scroll. Thus, instead of jarringly stopping at the edge without any indication that a boundary has been reached, the document keeps scrolling with your finger.

Windows Phone, however, does not show an "area beyond the edge" on over scrolling; instead, it anchors one edge of the document at the boundary, and subtly "stretches" the rest of the document to follow your finger. The effect is like trying to pull down an elastic sheet of paper. This indicates that the boundary has been reached, but without showing a separate "area beyond the edge". As such, it achieves an effect as intuitive as iOS's, but without infringing the claims.

(Yes, you really have to be the pedantic when evaluating claims.)

I'm pretty sure this effect came about because Microsoft wanted to work around the Apple patent. However, some sort of cross license, if not the one from the 90s, is apparently in place as well. Some UI elements in WP do infringe other Apple patents, such as the "disappearing scroll bar" one.