It makes sense really, both are designed for ARM architecture and neither needs the crutch of the desktop, based on the expected demographic of the typical buyer.
As phones grow and tablets shrink its logical that they should share the same OS - Apple amply demonstrated that with the iPhone and iPad. The only surprise is that lesson went so far over Microsoft's head with its original plans for RT.
The purchase of Nokia should really be an opportunity for Microsoft to push this merger ahead, after all in its current form RT is looking pretty dead and buried. Merged with Windows Phone it may have a future after all.
Just as long as Microsoft is careful how it treats its current user base when handling the upgrade. They really can't afford another WP7 to WP8 style disaster.