Numbers from nowhere?

I'm just curious:

print,3.3,format='(f30.28)'
3.2999999523162841796875000000

Obviously float is only good to ~6 decimal places, but IDL will still
print out additional digits.  It seems that it does reach a point
where IDL will only print out zeroes.  I'm just curious - where do
these extra digits come from?  Does it come from rounding errors
converting binary to decimal?  Or does it come from some other
nefarious location?