The determination of the Caspian Sea as a lake or a sea is in dispute and under negotiation between the 5 littoral states bordering it. A previous agreement between the Soviet Union and Iran listed it as a lake but since the break-up of the USSR the new littoral nations have refused to acknoweldge the previous treaty. If the Caspian were a lake the resources (oil, gas, fishing) and area would be split evenly between the five but if it is a sea the borders would be drawn out from the borders of each nation which would mean Iran would have considerably less territory under their control. There are other concerns but this is the primary issue.
Incidentally, if water located permanently beneath a hard surface is to be included, then Salar de Uyuni in South America should be on there. Below a salt crust surface is a 10,582 sq. km pool of brine.