Figuring out APR

When they work out the APR of a mortgage for an initial fixed term, do they calculate on the basis of moving to a standard variable after the fixed period?

Suppose you move to a tracker (that is less than the variable) after an initial fixed term, surely this would reduce the APR? Equally, if you fix again, this increases the APR?

My understanding is that the APR is calculated using the fixed rate for the fixed period and the variable rate applying for the remainder of the mortgage.

So you are correct in what you are saying