Barrack and charm HP should already been shown. What isn't shown is formation and squad.
If you need an example
Those players who have a huge difference in HP to what it shows in the prematch screen likely have high HP units in their HP squad.
25% of a unit's total HP is added to the squad of the category they're in. So say if you have 5 units in the HP squad each with 10k HP each in it, you get +2.5k HP x 5 for a total of +12.5k HP to your main party members.