Bunkers vs Trenches...

[MisterMark]

In my Salerno Campaign I am playing the allies and have routed out an enemy unit that was occupying a bunker hex.  In looking at the hex info I'm seeing the word 'Bunker' and then -20% +10.  I do understand that bunker in lower case means the fortification has half it's defensive value and once restored to full strength will have a defensive value of -40%.  But what does the +10 represent? 

Also I have occupied a trench hex and have restored it to it's full defensive value of -40%... if the terrain is the same for both these mentioned hexes, am I to assume the trench is going to offer the same defensive protection as the bunker once it's restored to full strength?  If that is the case, does that not seem plain wrong?  Shouldn't a bunker offer more protection than a trench?

Can anyone please explain?

-Mark