Chazaka beats rov (Pesachim 9a)
The gemara on daf 9a says that when you find fruit from a talmid chacham you can assume he took off trumos and maasros because it's not merely a rov but it's a chazaka that he took it off. This implies that a chazaka is stronger than a rov. There is the general rule though that ruba v'chazaka ruba adif - rov is stronger than chazaka. Which is it?
The answer is that there are many different types of chazaka. You could probably divide it into many different categories. I saw that Encyclopedia Talmudis describes it as five different types so I'll base this on that. The terms though are my own so they may not be completely accurate:
1. Acquisition - this is the way to acquire certain things like land. If you do something to the land to show your ownership.
2. Status quo - this is the most common type of chazaka. If something was one way then we assume it remains that way. Like we can assume the eruv is up since we know it was up when Shabbos started. Of course certain things can weaken the chazaka or ruin it but in general we assume the status quo remains. Rov is stronger than this type of chazaka.
3. Common knowledge - you weren't at this couple's wedding but yet you assume they're married.
4. Habit - Once something happens three times (or two) then this establishes it as the normal way.
5. Probability - Something happens this way almost all the time so we can assume it happened this way. That's the type of chazaka we're talking about in our gemara. Talmidei chachamam almost always take off trumos and maasros right away so we can assume it happened here also. This really is a rov - just a really, really strong rov.