VAB weight calculation incorrect

I have just built a calibration rocket, using only 3 parts:

the QBE probe body (0.08 T)
the X200-32 fuel tank (18T)
and a Mainsail (6T)
basic arithmetic gives 24.08T, however, in the VAB i get: [IMG]http://img.photobucket.com/albums/v342/ambee69/screenshot202_zps271c31a5.png[/IMG] 23.58 T.