The gemara understands from the pasuk וספרתם לכם that there is an obligation on each and every person to count sefiras ha'omer. Tosafos contrasts this to the mitzvah of counting to the yovel year which is only a mitzvah on the beis din, and question whether it is an obligation on the beis din to verbally articulate the count with a bracha as we do by sefiras ha'omer. However, for the counting of a zava Tosafos writes that there wouldn't be a bracha since it can get messed up if she would begin to bleed. Tosafos seems to be saying that we wouldn't make a bracha on something that is not predictable and within a person's control such as the bleeding of a zava, but it would still be fundamentally the same as the counting of sefira and yovel - to verbally articulate the count. The Pischei Teshuva (Y.D. 196:4) cites the Shl"a who requires a verbal articulation of the 7 clean days of a zava. The Shl"a understands from Tosafos that the bracha isn'e made, but the requirement to count is fundamentally the same as the counting of the omer and yovel. However, the Node B'yehuda argues as does many others, that the counting is fundamentally different and wouldn't require a verbal articulation. The Sidrei Tahara (196:18 d.h. mihu) writes that although one doesn't need to verbally articulate the number of days, they must have a concious thought of the number of days and consider them to be counting toward he clean days.The question is, what is the indication that the counting of a zava should be fundamentally different than the counting of sefira and yovel? To answer this question, the Sidrei Tahara quotes the Re'ah who writes תשובת דבר זה מה שהקדמתי לך בראש ספרי כי כל ענין התורה תלוי בפירוש המקובל. Meaning, we must follow the oral interpretation and cannot figure it out from the text. He points out that we find a similar discrepancy regarding the term זכירה. By yetzias mitzrayim we have a tradition that it must be verbally articulated but not by miriam. The Sidrei Tahara himself suggests a hint in the pasuk for the distinction of a zav and zava. By a zav and zava it says וספר לו and וספרה לה - why would the Torah have to point out that they are counting for themselves? It must be to teach that they are trusted on the bedika, rather than demanding a verbal count. The Meshech Chochma (Emor) has a different approach to explain the difference. Normally when we refer to a count, the purpose is to separate the days that are counted and infuse them with a special status. For example by a zav and zava it is a status of clean days that is acheived by being clean, it is not automatic. However, in the context of sefiras ha'omer and yovel where the days pass automatically and there is nothing specific to do, the requirement to count must mean a verbal count because there is no other possible demand. This explains the pasuk of וספרתם לכם ממחרת השבת וכו but the pasuk of תספרו חמשים יום is more similar to the "count" of zav and zava by giving that day a special stature, and not demanding a verbal count. תספרו חמשים יום means that day 50 is yom tov and is distinct in it's nature than the days that proceeded it. With this we are able to easily answer Tosafos question (d.h. kasuv) - why do we only count 49 days and not 50? The answer is that whenever we can explain the count to refer to a special status i.e. clean days or yom tov, we don't assume it requires a verbal count. The Meshech Chochma also explains the gemara on the bottom of the page - ואם תאמר ממחרת שבת בראשית, פעמים שאתה מוצא נ"א ופעמים שאתה מוצא נ"ב... meaning that sometimes the special day which is the "counted" day, will not be on day 50, rather anywhere between 50-56.