Two things. First, Google's Keyword tool does not disclose all of its data.

Second, the information it does display is a monthly average. So if you have a seasonal keyword the average per month is probably low. Something like a keyword to do with BBQ grills probably does not get many searches year-round, but during certain months it will attract more searches. The average though is too low to give any data.

Or if you have a keyword for something relatively new... the latest hot gadget... Well, there may not have been many searches for that product or keywords related to that product over the past year, so Google will have no data to display. It does not predict future searches, just reports past information.