determination of air fare

I go through internet to know the air fare from Kolkata to Portblair but observed that the fare is changing day by day. I am not aware how the air fare is determined. So I would request anyone who is well conversant about above may inform me the policy how the fare is determined. Example of rate of air fare which has been observed is given. On 24th June when I search through internet I saw that the fare including all is more or less Rs. 5435/- but when I observe the same on 27th June I saw that the fare is shown more or less 8000/-