According to my calculations 599.97 is equally divisible by 3:
599.97/3 = 199.99

All our prices ends in .99 but in a case when product is between 249.99 and 999.94 we offer 3 monthly payments plan. Therefore when Purchasing enters those prices system needs to tell them what number should they be using:

EX:

User enters: 599.99
System validates: 599.99/3 = 199.996666 NOT GOOD
599.98/3 = 199.99333 NOT GOOD
599.97/3 = 199.99 OK

Daniel