From the Trada website:

The majority of Scandinavian window manufacturers no longer use tenon
joints for the sub-frames. The jambs and mullions are end cut to match
the sill profile, but the connection often consists of a glued joint
and long screws driven through the sill from the underside into the
ends of the vertical members. This simplifies manufacture and reduces
the length of timber necessary for the vertical members because of the
omission of the projecting tenon. These screwed connections are
sufficiently strong for handling, and once the sub-frame is installed
and fixed, there is little strain on the joints.

http://www.trada.co.uk/techinfo/asse...sign%2Cdesigns