The only way this makes sense to me is if memory usage was reduced by a factor of 1.2, but that sounds so much more modest than "120% less" that I'm doubting myself.
On a related note, I tend to avoid expressing changes as percentages entirely, in large part because increasing by X% and then decreasing by that same X% doesn't get you back to where you started.
On a related note, I tend to avoid expressing changes as percentages entirely, in large part because increasing by X% and then decreasing by that same X% doesn't get you back to where you started.