UDP checksum calculation - uses lenght twice ?

I had some trouble understanding the UDP-packet checksum calculation, as is
described here : http://www.netfor2.com/udpsum.htm .

For some reason the checksum uses the size of the packet twice : once in the
real UDP packet, and once in the pseudo-header (containing the sending &
receiving IP, as well as the protocol).

It made me think I overlooked/did not understand something ...

Does anyone know why this is done ?

Regards,
Rudy Wieser