How accurate is the estimated search figure ?

When G suggests a keyword.

[Essex_boy]

Only ive a term that G says gets searched for around 18000 times month with only 3 other advertisers, however my account when the term is introduced says ill only get 3 clicks a day despite being top...

Which is odd to say the least, any ideas ?

[incrediblehelp]

From what I can tell it is not very accurate.