The difference is the aspectual character of the situation.
In the first, there is a steady state of being married, hence for.
In the second two examples there is a progressive change becoming worse (presumably continuously in small increments) totaling up to a noticeable change, and instances of amounts of rain falling totaling up to a considerable amount in a certain timeframe, hence during or over.
It's the interpretation of the situation that determines the choice of preposition.
We could use the same verbs and switch the prepositions.
We've been married three times over the past seven years.
Things have become noticeably worse for the past two or three months.
A considerable amount of rain has fallen for the past two years.
The steady state in the last sentence being a considerable amount of rain falling every year for the past two years.