The reasons for the extra players maybe down to provisional list that were sent to FIFA in advance of the tournament before final lists were submitted after all the teams arrived.
As far as I can tell, three of the players listed on FIFA's records in Peru's squad, did not travel to Montevideo: Juan Alfonso Valle, Jorge Sarmiento and Jorge Gongora.
Peru coach, Paco Bru, picked an 18-man team for a World Cup warm-up against Paraguayan team, Olimpia, on June 19th 1930. The list was published by Peru's El Comercio on June 18th and included Sarmiento and Gongora but no Alfonso Valle. Arturo Fernandez and Luis Souza Ferreyra would be added to the squad before departure.
The list below is from A History of the World Cup, vol 1. The Jules Rimet Years (1998);
Juan Valdivieso (Alianza),
Jorge Pardon (Atletico Chalacao),
Alberto Soria (Alianza),
Julio Quintana (Alianza),
Arturo Fernandez (Universitario)
Domingo Garcia (Alianza)
Alberto Denegri (Alianza)
Eduardo Astengo (Universitario)
Placido Galindo (Universitario)
Pablo Pacheco (Universitario)
Carlos Cilloniz (Universitario)
Luis Souza Ferreira (Universitario)
Mario De las Casas (Universitario)
Julio Lores (Necaxa, Mexico - registered to the Peruvian FA for the tournament)
Antonio Maquilon (Atletico Chalacao)
Jose Maria Lavalle (Alianza)
Alejandro Villanueva (Alianza)
Demetrio Neyra (Alianza)
Lizardo Nue Rodriguez.(Alianza)
As of yet I have been unable to obtain any other contemporary source that includes Juan Alfonso Valle, Jorge Sarmiento and Jorge Gongora.
The 18 man team in El Comercio, 18 June 1930, can found in the article below.
|FIFA's match report Argentina v USA in 2003 show two extra US players|
|FIFA match report Uruguay v Romania in 2003 shows five extra Romanian players|