The rule about ritual purity is learned from the law of suspected wife: if a husband tells his wife not to seclude herself with a specific man, but she nevertheless goes into seclusion with him, then they need to go to the Temple, where she drinks the bitter water, to prove her innocence.
This is a situation of doubt: it is not known if she was unfaithful or not. Since seclusion is only possible in a private place, we have a rule: any doubt about purity that arose in a public place is declared pure, and in a private place it is impure. For example, if dead mole was found next to a piece of sacrificial meat, so that it might have touched it, then if this happened in a public place, the meat is pure, but in a private place it is ritually impure.
If there was a mikveh, which initially had forty seah of water (about two by two by six feet) but then was found lacking the complete amount, the purity of all objects that were dipped in it is now in doubt. First of all, this rule seems to contradict the retroactive impurity rule we learned before. Secondly, in this case the Sages declare the objects impure – and compare this to suspected wife, who is forbidden to her husband until she proves her innocence. Rabbi Shimon also compares this to suspected wife, but derives that the objects in the public domain are pure. This leads to seeming multiple contradictions, and the Talmud resolves all of them.
Art: Georges Seurat - Seated and Standing Woman