Why under employment is also known as disguised unemployment?

Underemployment i a situation where people are working but they are working less than their actual potential like no other option of employment is available, in this case, labour efforts are divided. This kind of employment is hidden in contrast to someone who does not have a job, which is clearly visible and called unemployment; it is also called as disguised unemployment. For example, a family have a 1 acre of land, employing a couple, husband and wife, now they have a son, who get married but do not have any other job, so now the same piece of land is also employing them