Not a theory :) The classic example is with scientific facts, things that are always true no matter what the time. So if the doctor says "Water freezes at 0 Degrees C" I report that as "He said water freezes at..."
Although technically (and after a quick search I see that there are many good threads on this forum that go into detail on this) this is a difference between the way we are supposed to speak and the way we actually speak. The very very strict grammar is to always backshift, but in practice people don't do that and from time to time backshifting when the situation you are describing is still true can lead to confusion. That confusion usually disappears in the course of conversation because people don't just report random sentences they use reported speech in a story or explanation.
There are some teachers (and students) who want to be very strict about grammar rules, and I respect that (even agree with it sometimes) but there are lots of times when the rules do not live up to everyday English. After all when I was in school we weren't supposed to split infinitives, yet because everyone does it all the time that's no longer taught as a grammar rule anymore.