What is the significance of the similar number and arrangement of bones in a human arm and a bat wing?

It shows that bats are more closely related to humans. Scientists suggested that bats evolved from non-flying mammals that live on trees like lemurs and squirrels. Some of the members of the early species of bats probably have a mutation  which causes more skin extending between their arms. These particular bats with mutations had better mobility than others helping them thrive and reproduce, and eventually evolved through natural selection retaining the wider skin membrane. This, in turn, became full functional bat wings.