Adiyen has a doubt in Prabandasara pasuram on Periazhvar. Swami Desikan while mentioning the count of pasurams, at the end say four hundred and seventy-one and two instead of four hundred and seventy-three (“nAnURRuezhubathonRiraNDum”). Is there a reason behind this?

Generally, a pasuram or poem is written based on the kind of verse (viruttham); composers construct the words based on the type of viruttham. As this is based on En Asiriya Viruttham, he has mentioned four hundred and seventy-one and two.
Notes:
An interesting aspect – from Periazhvar Thirumozhi, the first two pasurams ‘pallandu pallandu and adiyomodum ninodum’ are unique in that these two pasurams are chanted in the beginning of recitation during nitiyAnusandhanam.
Though these two are separate pasurams, it is a tradition that two of these pasurams are considered one.
“nAnURRuezhubathonRiraNDum” – though Swami has set this pasuram in Chandas metre, the fact that he is included a special meaning shows our Kavi Simham’s singular ability.