Most strong electrolyte solutions are not ideal so that the measured value of the van't Hoff factor i is than expected from the formula of the compound. This is because ions in solution are not totally isolated from each other, but remain clustered together to form an ionic atmosphere. This the effective concentration of particles and therefore the measured value of i.

Van't Hoff factor, i is less than expected from the formula of the compound and this decreases the effective concentration of particles.

Explanation:

Strong electrolytes are defined as the solutions which completely dissociate into their ions.

But, most of the strong electrolytes do not dissociate completely (act ideally) but remain partially undissociated into the ions. Thus, giving a lesser number of ions in the solution than expected.

This means that the Van't Hoff factor, i which denotes the number of ions is less than expected.

As fewer ions are released in the solution, the concentration of the solution is expected to decrease.

Hence, van't Hoff factor, i is less than expected from the formula of the compound and this decreases the effective concentration of particles.