They aren't. You will notice there was a 1984 mentioned in the thread. One reason the others were 1975 is if you notice the tittle of the thread has 1975 in it. If someone has a silver colored cent they tend to do a search for silver cent or silver penny with the same date as the one they have. They find such a thread and ask about their coin in that thread. If you just search for silver cent or silver penny you will find threads of many different dates.