I am doing some Latin exercises and the sentences are given as the following (by Rosetta Stone):
Puer plus lactis habet quam vir.
Femina plures canes habet quam vir.
Puer plus lactis habet quam vir.
Femina plures canes habet quam vir.
In the first case ("more milk") the object is in the genitive, but in the second case ("more dogs") the object is in the accusative. Why would these two parallel sentences differ in this way?