Flip a coin 10 times, you can get any number of head and tails (adding up to 10 of course). But it is possible to get 10 heads and 0 tail (or the opposite).
But flip the same coin 500 times and you will get 250 head and 250 tails (at worst, 248-252, but the difference will not be any bigger).
bottom line, if you do something for long enough, it eliminates the 'chance' factor from the equation. how that translates to cricket? in T20, anyone can win by chance, not necessarily the best team will win the match. In test, with the chance factor eliminated, the best team will win. If there is no significant difference, it will be drawn.
That is the root difference between T20 and Test (BTW, this is where the name Test came from, a test of real quality)
Ekona tomar chena rastai, kagoj kurea voray bostai....