Question

Have a good week, Rabbi,
In tractate Pesachim 9a, there appears a series of doubts regarding the search for leaven. The second doubt mentioned is:
Two piles, one of matzah and one of leaven, and before them two houses, one that has been searched and one that has not been searched. Then two mice came, one took matzah and one took leaven, and we do not know which entered this house and which entered that house — this is the case of two baskets. As we learned in the Mishnah: Two baskets, one of ordinary produce and one of terumah, and before them two se’ahs, one of ordinary produce and one of terumah, and these fell into those — they are permitted, for I say: the ordinary produce fell into the ordinary produce, and the terumah fell into the terumah.
And the fourth doubt mentioned is:
A doubt whether it entered, a doubt whether it did not enter — this is the case of a valley, and it depends on the dispute between Rabbi Eliezer and the Rabbis. As we learned in the Mishnah: One who enters a valley during the rainy season, when there is impurity in a certain field, and says: I walked in such-and-such a place, but I do not know whether I entered that field or whether I did not enter it. Rabbi Eliezer rules him pure, and the Sages rule him impure. For Rabbi Eliezer would say: a doubt about entering is pure; a doubt about contact with impurity is impure.
My question is: what is the difference between these two types of doubt? After all, in the second case too we ultimately have a doubt whether it entered or not entered (and therefore it would seem comparable to the valley case), and in the fourth case one could also be lenient as in the case of two baskets and say, “I say the mouse did not enter” (and according to the Sages as well).
Best regards,