How does the Talmud arrive at its interpretations of these pesukim? The answer is to be found in the pesukim themselves. Whenever we find the word “lachem,” the plural form of “you,” the implication is something that is incumbent upon the entire house of Israel – each and every individual. Whenever we find the word “lach,” the singular form of “you,” the implication is something incumbent upon a particular entity in the house of Israel – namely bet din.
Thus, the word “lach” is used regarding Yovel since it is bet din’s function to designate the Jubilee Year. Regarding a person who has an impure discharge (Leviticus 15:13), counting the days toward purification obviously cannot be the function of bet din (although the singular form of “you” is used); it rather is that of the particular impure individual. Tosafot (loc. cit, s.v. “U’sefartem”) explains that the impure person doesn’t say a berachah because he might have to start his count all over again if he sees a fresh discharge.