Hey thats a great theory, and gravity definately isnt a constant because it depends on the distance between the two bodies, so if I hold my standard of mass a little higher the gravitational forces will be less, and therefore it will appear to weigh less !
This is pure speculation and I am in way over my head on this one !
Alternatively of course it could be that the earth itself is losing mass - global warming, ice caps melting, life on mars - it all makes sense now ha ha ha ha !!!!!!!!!!!!!!!!................
er more coffee please