Your way of thinking is completely logical, but it does not harmonize with the algorithm given by theymos - he clearly wrote that he uses the function of rounding up in other case  he would use a simple rounding function (as you described). I am also convinced that after receiving my first positive feedback I did not have to wait two weeks and trust score appeared immediately (that means, the rounding up function worked).

Therefore, I think that the algorithm works, while the trust is recalculated not continuously only at a specific time (for example on a particular day of the month), except the situation when someone receives a positive /negative feedback - then the trust score is recalculated immediately.