If you like, think of it in terms of money.
When you have $500 in the bank, if it goes to $250 suddenly, you have less money; your wealth - which is what the balance is a measure of - has DECREASED.
The wife (or SO) keeps spending cash, and soon you have $0 in the bank; your wealth decreased again.
Now she keeps on buying shoes, and soon you have even less wealth - -$250 is the balance.
And if she keeps buying trinkets, your balance will become -$500 - the raw number got bigger, but because of the "-" you didn't get more wealth - you got less.
thats all thats going on - AoA happens to be a negative number (due to the way the vanes are calibrated) and the 'normal' way of looking at increase/decrease by looking just at magnitude doesn't work.