Why does my Conn 14k sousaphone go over 40 cents sharp when cold?

My school has 6 Conn 14k sousaphones having 2 with a large wrap 2 with a medium wrap and 2 with a small wrap. I use one of the large wrap sousaphones and while the other 5 have the tendency to be flat when cold the one I use is always extremely sharp when cold and neither my music teacher nor I can figure why or how this is happening, so I am asking if anyone knows why the sousaphone I use go against the natural tendency all wind instruments have including those of the same model, age, and everything else since my school has had all 6 of these Conn 14k's for about 50 years and yet only one is like this. And yes this sousaphone acted like this before I started using it.