One of the characters "inherited" it from her grandmother (read: the grandmother's name is still on the lease and the landlord was never notified) so the origin

One of the characters "inherited" it from her grandmother (read: the grandmother's name is still on the lease and the landlord was never notified) so the original lease was probably signed a long time before the show's timeline begins. Since rent control limits rent annual increases, that means they're paying well below market rent.