Varients of this question come up a lot. For example, it was recently discussed in this thread:
That thread has links to several resources that answer this question, including
- the first-ever thread on the Answers forum that addresses this question
- a nice FAQ that includes an answer to this
- a "Cleve's Corner" article that talks about the limitations of floating point
and well as some nice comments, within the thread itself, about the difference between decimal and binary representation.
I would note that in your example, the discrepancy is not in the 5th or 6th decimal place. It is in something more like the 15th or 16th.