Our perashah predicts abundant blessings as a result of keeping the misvot. The Talmud says that the word 'asamecha - in your storehouse' is related to the word 'samoi' or hidden. The Talmud teaches that blessing is more likely to come if it is hidden. If something is counted, measured or weighed, it is not likely to miraculously multiply.
Open miracles are hard to come by.
However, Hashem is more likely to perform hidden miracles. Therefore, if you don't really know how much you have in your storehouse, Hashem might multiply it for you. Torah says it is the same thing regarding the observance of misvot, the source of blessing. A person really can't perform misvot easily if he makes too many calculations. If you keep weighing all your options, the yeser hara, man's evil inclination, will involve himself in your business and stop you.