there is achance for me to guess a key,no matter how small that chance is it do exist
Sure, the chance of you guessing a previously used private key is non-zero, but it is so unimaginably close to zero, that it is effectively zero for all intents and purposes.
freebitco.in for example,the probability of winning each time you bet hi is 5/10000 that is 5^-4 compare to 1.4^-47 the diference is 10^-43,if a computer check this adresses everyday,5 month is enought to spot at least one or two used adress.I think you are exagerating a bit the results.consider this as an ordinary probability problem.