Hippy,
I might just be spiffballing here but I believe many controllers add the term 'report level' to avoid potential ambiguity. If an ac at 5000ft is told "descend 2000ft", will the ac level at 3000ft (5000 - 2000) or at 2000ft? If an ac at 5000ft is instructed to " descend, report level, 2000ft" surely there is little doubt as to where the ac might end up? As controllers (and hopefully pilots too) we all know where we expect the ac to level but I believe the addition of 'report level' helps prevent such misunderstanings?