Fixed interest loans have the interest remaining the same for the whole duration of the loan. While it may seems to be an advantage to have a fixed interest rate, it is in fact a disadvantage because the interest rate is set not at the medium or half way between the maximum and minimum possible rate, but at slightly above that. So that for a majority of time you will pay more than the official bank rate. This also remove the advantage of paying at a low interest rate when the official lending rate falls.
The variable rate is always a few percentage points above the official lending rate. The interest will then vary with the official lending rate. The disadvantage of having a variable rate is that in time of high interest rate the loan can be difficult to service.