What cause the ratio of PV/RT to drop below 1 when 1 mole of
gas is gradually subjected to higher and higher pressure?

pressure and temperature are directly proportional.

Explanation:

At constant temperature and pressure the volume of a gas is directly proportional to the number of moles of gas. Also given a constant temperature and volume the pressure of a gas is directly proportional to the number of moles of gas.

This means that if V is constant then

P = n (RT/V) then n= PV/RT. As P is increased, T is also increased thereby decreasing the value of n since pressure and temperature are directly proportional.