A hole and two boxes

Have a good week Rabbi,
In Tractate Pesachim 9: a series of doubts appear regarding the testing of chametz. The second doubt that appears is:
Two public places, one for matzah and one for chametz, and in front of them were two houses, one checked and one not checked, and with it two mice, one shekel of matzah and one shekel of chametz, and they did not know that it was a shekel of matzah and it was a shekel of chametz – that is, two boxes. They said: two boxes, one for khul’in and one for teruma, and in front of them two s’ain, one for khul’in and one for teruma, and these fell into these – it is permissible, as I say: khul’in fell into khul’in, and teruma fell into teruma.
And the fourth doubt that appears is:
Doubt about doubt not about – we mean the valley, and in the group of Rabbis Eliezer and Rabbis. They said: One who enters the valley during the rainy season and becomes unclean in such and such a field, and one says: I walked in such and such a place, and I do not know whether I entered that field or not. Rabbi Eliezer purifies, and the sages defile. Rabbi Eliezer used to say: If one enters, one is pure, one is unclean, one is unclean.
I asked what the difference is between the two types of doubts, because ultimately in the second case too we have a doubt whether it is on or not (therefore likening it to a gap), and in the fourth case too we can hang on to the sound as if it were two boxes and say “I say a mouse, not on” (also likening the sages).
Best regards,