oddly enough i logged in to ask exactly this question because it was my tank stumpy was trying to murderalise

Sadly you are mis-remembering Lex.
The actual rules for Steam Tanks say this:
"Because of its exceptionally heavy armour plating, a Steam Tank always counts as defended - so a 5 or 6 is normally required to inflict a hit from shooting or in combat." Cannons on the other hand state this:
"Cannons ignore a target’s Armour because no armour can nullify a hit caused by a cannon ball. No Armour roll is made. In addition, fortified targets only count as being defended (5+ to hit) and defended targets count as being in the open (4+ to hit)."This, unfortunately, is entirely contradictory and I can see the case for either.
Basically speaking should the tank counts as defended not because it is behind cover but because it is so hard to find a weak spot to cause damage through and thus the to hit penalty is imposed to reflect the difficulty of hiting such weak spots or to put it another way... do the rules say it counts as defended because the -1 to hit already exists in the rules under this concept and it was supposed to keep things simple or...
Should the cannon just blow the crap out of the tanks uber armour cos, well, it's a freaking cannon man and nothing but nothing can hide from the cannonballs of doom! I can completely see this being the case... it is after all a freaking cannon that can blow holes in fortifications but both interpretations make sense to me and i wonder what others think?
One last point, the Steam Tank rules were adapted to include the 'always defended' bit considerably after the cannon rules were finalised... is it just a case that someone forgot that cannons did this?