Although yarns can be categorized, it doesn't mean that yarns in the same category will have the same gauge.
Gauge can be affected by the "loft" of a yarn (fuzziness). Take mohair for instance..what looks like a fairly thin strand of mohair, will knit up as a worsted simply because of "loft".
Swatching to achieve the gauge called for in the pattern, is the only way to be sure..though this can also result in needing MORE yarn than the pattern calls for.