Reasons for the difference in design patterns for mobile to desktop vs. desktop to mobile

I want to follow up on a recent question that related to the WeChat Web experience but expand the scope of the question a little bit further: Is WeChat Web bad for UX?

In recent times companies that started out as smart phone applications (e.g. IM applications like WeChat, WhatsApp, LINE) have expanded their offerings to create web applications that require users to trigger the login process using their phones (e.g. QR code scan or entering token). This is in comparison to desktop applications that started out as desktop or web applications (e.g. Dropbox, Gmail) that have created smart phone applications using similar login processes.

Are there specific design patterns or guidelines that provide designers with a consistent way to design the experience for transitioning between platforms (i.e. phone, desktop, smartwatch, etc.) and the behaviour of the applications? Are there any specific reasons for these differences (e.g. security or cultural) since the user experience tends to be quite different even amongst the major players in the market like Google, Microsoft and Apple?