Here is a little more background on the update that delivered a Windows 10 style Start Menu to Windows RT 8.1 systems



From Supersite for Windows:

It turns out the KB3033055 update brings two Windows 10 centric elements to Windows RT 8.1.

One of the feature improvements, the Windows 10 style Start Menu, is one of the most visible aspects of this update. You will notice however, that scrolling is horizontal on the RT Start Menu compared to desktop version of Windows 10 which is vertical. This is due to the types of controls used on the desktop version of Windows 10 not being available on Windows RT 8.1.

  What's new and improved on the Windows 10 Start Menu on Windows RT 8.1