It's a little more consistent, all those As look more aesthetically pleasing, and if your university offer was AAA, it'd eliminate the possibility of being rejected for not meeting it, which could happen if you had A*AB.
A*AB, For engineering I like to specialize, in this case A* in maths is better then a A in maths. Even you got something like a B/C in another subject that you don't need. e.g French (No offence to people that take french)
AAA. An A* is lovely but because it's gained through meeting certain conditions after already having an A grade instead of a straightforward % of UMS it's sort of…not another whole grade up. Unlike the B, which is a whole grade down.