The "RT" In Windows RT has no official meaning according to Microsoft but there is a general use that suggests its origins.
RT does stand for Runtime, which has been in long time use – long before Windows 8 – so it is natural to think there was some sort of natural progression internally at Microsoft from the early days of referencing Runtime to a shorted form of RT to WinRT to the current naming of WindowsRT.
To confuse matters more, WinRT is the formal name of the APIs used to develop Windows 8 apps and programs, as in WinRT APIs (not WindowsRT!). These APIs let developers code once for programs that would work on both the ARM processor-based and Intel processor-based versions of Windows 8.
So WinRT does not directly translate to WindowsRT or vice-versa – there is just a hint of meaning referencing each other.
It could also be a clever play on "real-time" meaning the operating system is for light-weight OSes running on phones and tablets, but once again, this is not an official naming.