Clear zone is obtained when the bacteria on the agar plate are killed by antibiotic. This takes place at a certain controlled temperature. and the action takes place when the amount of bacteria are controlled. When both the variables are not controlled i.e if the heat is increased then antibiotic will not act on that high temperature or when the amount of bacteria may be increased or decreased then the antibiotic action will not occur leading to change in the clear zone.
If antimicrobial agent leaches from the object into the agar and then exerts a growth-inhibiting effect, then a clear zone (the zone of inhibition) appears around the test product.