The number of decimal points for single precision is 6-9. What this means that single precision number can always represent at least 6 decimal digits. And that when you print it, you have to print at least 9 to not lose accuracy.
For double precision it is 15-17. So double can always represent at least 15 digits. And when you print it, you have to print 17.
It comes from the fact that 6 decimal digits is sometimes not enough to represent the number exactly. Not always, but sometimes you need 7, 8 or even 9. But not more than 9.