I'm predicted AAB And a uni I applied is AAA-AAB, they've offered me BBB! Why would they offer me lower?
Don't worry chap, that is good news! University's are more likely to give lower offers to students they want at their institutions as it makes it more likely for you to accept that offer. Good job on that!
Some unis say their typical offers are higher than they actually are to make themselves look more prestigious. They could also have had a shortage of good applicants this year or are confident about your ability.