[hanimos]

It's a good idea, and worths being experimented in our schools and classrooms
I personally think that it has to do with - related to - the psychological side,and can be explained at the affective level, because students may tend more to preserve what they already have -20- than trying to have it -20.
That's to say, good students are well motivated in advance due to different factors such as cognitive and affective abilities and tendencies which result from their socio-economic baground ;
whereas 'bad' and lazy students are frustrated and don't have the will - not the ability- to work hard in order to achieve good results while learning - due to the same factors stated above; thus the fact that students already have 20 /20 in hand stimulates their will - motivates them - and makes them ready to preserve what they already own, according to the saying

A bird in hand worths thousands in the bush.

NB- Sorry for the use of English , I'd rather write in Arabic but I haven't installed Arabic yet