Difference with Google WMT

Hi,

I found out that the counts of organic searches from Clicky differ from what is reported by Google WebMasterTools (WMT).

Even when I count only unique IP's and focus on google-searches there is still a difference. For monday, for example, one of my sites counted 69 according to Clicky, but only 61 according to WMT.

Does anyone know why WMT reports less?

Thanks
Bas.