Yevamot 119B

רישא, חזקה לייבום, ורובא לשוק. וחזקה לא עדיף כי רובא. ואייתי מיעוטא דמפילות סמוך לחזקה, והוה ליה פלגא ופלגא ⁠— לא תנשא ולא תתייבם.

In the first clause of the Mishnah, the legal presumption is that this widow is obligated to enter into levirate marriage, but in a majority of cases she will actually be permitted to marry a man from the general public, because it is statistically probable that her rival wife had a child. A legal presumption is not as significant as a majority, meaning that the majority carries more weight than the presumption, and she should be permitted to marry immediately. But bring the minority who miscarry into consideration, and join this to the legal presumption, and it becomes similar to an even balance of half and half. Those who miscarry detract from the strength of the majority, causing it to be equal in legal significance to the legal presumption. Therefore, the ruling is that she should not marry any man who is not her yavam and she should not enter into levirate marriage either.

סיפא, חזקה לשוק, ורובא לשוק. והוי ליה זכרים מיעוטא דמיעוטא, ומיעוטא דמיעוטא ⁠— לא חייש רבי מאיר.

However, in the latter clause, the legal presumption is that the widow is permitted to marry a man from the general public, since her late husband had no brothers initially. And in a majority of cases her mother-in-law will not have had another son, and therefore the widow will actually be permitted to marry a man from the general public. Consequently, the possibility that her husband has a brother, necessitating levirate marriage, is not taken into account because it is a minority of a minority, i.e., it is a minority and it contradicts the legal presumption, and even R' Meir is not concerned about a minority of a minority.