Posting since there is a bias on this site toward posting higher scores, which makes it look like any idiot can break 70%.
I've noticed the same trend but I wouldn't worry too much provided that the rest of your applicant profile is strong. My score was also 660 51% but I'm not terribly worried. General GRE scores are 170Q 157V and 6.0AW. The subject test doesn't measure your ability to study in a top doctoral program. It measures how much you remember from your undergraduate program or how much you prepped with outside materials
My BA is in mathematics from a pretty well known liberal arts school (graduated in 2008 with 3.74 GPA) and my MS in mathematics and statistics is from a top 25 school (private univeristy in DC - graduated 2010 with 3.81 GPA in concentration). I'm applying to several top 15 programs in applied mathematics and computer science and am happy with my overall chances. I've spent the last 6 years teaching high school mathematics at an IB world school and did adjunct work as a stats professor. I haven't encountered several of the topics on the exam in over 8 years so to score 660 to round out my application should be fine. Work on your SOP and get some good letters.
As one of my advisors once told me, good doctoral programs want to know that you are willing to learn and be challenged. Provided that you have a good background, they'll teach you what needs to be taught.