The terms ‘Ishan’ and ‘Ishvaran’ denote Sriman Narayana as per Vaishnavam, why had Azhwars-s mentioned the word Ishan while referring to Lord Siva in their pasurams?

In Saivam the word ‘Ishan’ denotes Siva as their God, but on the whole, Ishan refers to Perumal and as a part of speech Azhwars had mentioned Ishan denoting Lord Siva.