It is due to the effect of temperature on our taste receptors.
Explanation:
The taste receptors on our tongue is extremely sensitive.
It can vary the taste of same element due to difference in temperature. So cooking further, the temperature of that food is increased which makes it taste more saltier than before.
The solubility may also have the effect on this case because solubility increases with the increase in temperature. So at high temperature salt may expand and dissolve more to make the food saltier than before.