When it says something like AAA-ABB it usually means that ABB is the lowest offer they will give and it's usually reserved for students from a disadvantaged area or 'widening participation' school, or someone with extenuating circumstances. The standard offer will be AAA.
However if you have AAB at AS I would definitely apply as it's your predicted grades that matter for getting offers and as long as you have a good reference and personal statement the university will think it's perfectly possible for you to raise one grade in one subject for A2. Plus, depending on the course, some universities will SOMETIMES let you in on results day even if you don't get the grades so it's worth getting the offer and then seeing what happens. As long as you make sure you apply to other universities with lower grades and have a good insurance option, it's good to aim high.