Even after 1000 submissions there will be chance to win assured vouchers because in those 1000 posts there are posts by individuals who have crossed their 10 valid entry per blogger mark.
So that means 1000 valid entries eligible for voucher is the criteria for assured voucher.
So out of the 906 posts there are 120+ posts by the blogger who have already crossed their quota of 10 valid entries.
eg.- 27 posts written means 17 extra posts in top 1000 in the same way 13 posts means 3 extra posts.
so summing the extra posts the result comes out to be 120+.
Lets keep our fingers crossed that the logic works fine.