thats how IndiGo business model works...out of the 30 odd existing fleet only about 4-5 are owned by IndiGo.........IndiGo(Interglobe) sells the A320s to leasing companies (makes a premium there) and leases it back for the airline...I guess when the 5 year lease ends, IndiGo will not renew the lease on those airplanes but would want to lease brand new A320s from the same/other leasing companies keeping the average age of the fleet very young...moreover, only a small number out of those 180 will come into the airline...most of them will be leased/sold to other airlines...
google