Statement 1 : ${\mathrm{HClO}}_4$ is a stronger acid than ${\mathrm{HClO}}_3.$

Statement 2 : Oxidation state of Cl in ${\mathrm{HClO}}_4$ is +VII and in ${\mathrm{HClO}}_3$ +V.

Both statements are true and statement 2 is not the correct explanation of statement 1. Greater the number of negative atoms present in the oxy-acid make the acid stronger. In general, the strengths of acids that have general formula ${\left(\mathrm{HO}\right)}_{\mathrm{m}} {\mathrm{ZO}}_{\mathrm{n}}$ can be related to the value of n. As the value of n increases, acidic character also increases. The negative atoms draw electrons away from the Z-atom and make it more positive. The Z-atom, therefore, becomes more effective in withdrawing electron density away from the oxygen atom that bonded to hydrogen, in turn, the electrons of H - O bond are drawn more strongly away from the H-atom. The net effect makes it easier from the proton release and increases the acid strength.