FYI english is not my mothertongue so i may not understand some technical meanings. What do you mean by that "higher" ? Are you saying that the width of the platform represents the bottom of the hull (which naturally is narrower than the actual hull deck)?
Okay, I'll reference these diagrams.
First of all, we need to understand that the platform on which the mock up sits is not the actual hull deck height. E.g.: The aft of the mock up platform is not the height of the helicopter hangar. Instead, the mock up platform is higher than the actual hull deck height, and the only part of the platform deck which is representative of the real thing is the height of the main gun. The diagram below demonstrates. Note how the actual height of the true hull deck is lower, shown by the notional helicopter flight deck/hull deck height being lower than the platform.
That has been the design for most recent PLAN DDGs and FFGs (see 054A below), so it is likely it will be the design for 055 as well. Also, if the platform deck height really were the hull deck height, then it would make no sense because the hangar is too low to even hold a Z-9 let alone a Z-18.
So, what does this mean for 055's actual beam?
I will use a handy 052D diagram to demonstrate -- we're talking about 055 but the principle is the same. The greatest width of a ship (i.e.: its beam) is usually found at the level of its true hull deck (the black line). On the other hand, in the 055 mock up's case, the platform "cuts" the ship "horizontally" a few decks higher than the true hull deck line, at about the red line.
Now, notice how the geometry of the ship above the true hull deck is like a triangle (green lines). So the further higher up you go relative to the true hull deck, the narrower your measured width becomes.
So, to quote you, you ask if the width of the mock up platform represents the bottom of the hull, correctly saying the bottom of the hull is narrower than the actual hull deck.
The answer is that I'm saying the width of the mock up platform represents a horizontal cross section a few decks above the actual hull deck -- and of course it is actually narrower than the actual hull deck as well.