When you do a price check on Google, it simply takes the USD price, and then does a conversion on what that would be in Rand based on the current USD/ZAR exchange rate.
Google is not going to any S.A. exchanges to look for the buy / sell prices or latest trade price.
The bitcoin exchanges in S.A. dont price bitcoin on what the USD price is in Rand, it is based on supply and demand, like any market. So if there is very high demand for bitcoin locally, then the price will go up as traders will increase their prices.
Doing a Google search for the price is a little misleading, kind of like when you do a Google conversion for USD to Rand, and then expect to get that rate at a bank when you exchange your USD, which wont happen.