[Q] Revenue and profit margins in PC vs Mobile

With the overall negativity in press regarding the current state of PC sales/shipments and windows ecosystem, and enthusiasm about the mobile side of things, I just read something surprising, and would need some clarification, if someone here knows.

So, Lenovo. Their shipments of phones+tablets have outgrown shipments of PCs. Yet, laptop division alone accounted for 50+% of overall revenue, three times as much as mobile division, according to the Lenovo press release. Now, I know that Lenovo mostly sells cheap (not flagship) products in mobile, where margins are thinner, but it's still surprising. Is it possible that margins in smartphones+tablets industry wide could be even smaller than for PCs?

If so, why did Sony pull out? Why is Samsung pulling out, kind of? Why have others contemplated pulling out? Why is there a constant pressure for MS to lower the price of windows license? Also, if so, it also makes sense to make WP free for OEMs.