Understanding Society User Support

I'm sorry, I've got another question on the variable "b_ptimings_ip" on IP2.
As in IP2, the times are given in seconds for individual questions. However, when I was looking at the data, among all cases of fully productive interview, there are about 900 cases with values for each variables (of individual questions) are shown as "inappropriate". When I checked against those individuals, they have answered some of the questions in the individual questionnaire, therefore, I would like to know why the times are shown as "inappropriate".

Thank you very much.