You could take it as, in order to make both clauses have a point :
5 Strength
Helmet, +2 AV, less than 5, so full bonus applies.
Ballistic Shield, +6 AV, more than 5, so capped at +5 AV
Total bonus is +7AV.
This exceeds strength by 2 points, so they'd get the -1 penalty (though I like the idea of making it based on the max rather than actual bonus because it still has that size!)
Of course, nothing we're discussing matches the example!