Halliday & Matthiessen (2004: 499-500):
The reality-phase, or realisation, system is based on the contrast between ‘apparent’ (seems to be) and ‘realised’ (turns out to be); both are perfective, the first being unreal, the second unreal emerging into real. …There is a variant of the ‘realised’ which is imperfective … ; this is looking at it from the ‘real’ end, as reality emerging from appearance. We can also relate the passive voice to this general meaning, with its original sense of ‘is (in a state of) having been realised’.