One who has a safek on a certain day if he counted sefira keeps counting with a bracha. Why don’t we say – there is a chazaka that he did NOT count, if he has no recollection of having counted and it is so easy to forget? The same way by other things we say there is a chazaka he did not say it (mashiv haruach, yaaleh v’yavo).
Good question, and let me add to the question. Here we are saying the when we have a sefek sfeika we do say the bracha, but in 215-2 the M:B says that we don’t say a bracha even when there are two sfeikos to say the bracha? See Halichos Shlomo- Pesach 11 ftnt. 24, that the main reason we can count the next night is because most Rishonim hold that even if one missed a night he can still continue with a bracha, as each night is a separate mitzva. In practice we don’t do this because of the opinion that it is all one long mitzva, therefore we don’t say the bracha. However when even the is a safek, then we are not machmir and he is allowed to count with a bracha.