Comment 4 for bug 282234

This is because in Kate the tab key indents instead of inserting a tab character. If your indentation width is smaller than tab width, Kate inserts spaces. If indentation width is larger than tab width, indentation is done with a combination of tabs and spaces.

You get the desired behaviour if you set the tab width to the same as the indentation width.

I agree that this is very confusing. One should be able to set 'indent with tabs' and indentation width would then be determined by tab width.