Why does lady Justice have one boob out? And why is this the first time I have noticed this!

So! In earlier art images were often coded, and had symbolic meaning that was very clear to the people of the time who viewed them, even today, though these things are meaningless to the undiscerning eye. Many have lost the code or don't care to learn or understand it.
A naked breast was exactly what it means today when we see a woman in the same condition (although for modern modesty she will tend to hide it). She is feeding her baby. She is bountiful; fertile. Therefore a woman in a painting with a breast hanging out, even if she is in fancy dress, is a symbol of life-giving, and of prosperity.