Researchers have widely adopted measures of teachers' mathematical knowledge for teaching (MKT). This paper investigates why teachers select "I'm not sure" as a suggested solution in MKT items. In this study, in-service teachers responded to multiple-choice MKT items, they submitted written responses to open-ended questions, and they discussed these items in group discussions. We analyse the different kinds of responses from three teachers in depth. A framework of cognitive types of teacher knowledge--distinguishing between Type 1, Type 2 and Type 3 knowledge--was applied in the analysis. We suggest that one teacher selected "I'm not sure" due to insufficient MKT, whereas the other two teachers appeared to have Type 1 and Type 2 knowledge. These findings provide proof that the knowledge teachers utilise in responses to open-ended questions and group discussions does not necessarily mirror the knowledge used when selecting a particular multiple-choice response.