Learning is a complicated task. This isn’t helped by how complicated it is to teach.
Students who watched a less confident lecturer predicted their own understanding of lesson better.
Meanwhile, a more confident lecturer made students think they understood the material better. In both cases students scored about the same. The students from the confident lecture overestimated the scores they expected to get.
Citation: Carpenter, S., Wilford, M., Kornell, N., and Mullaney, K. (2013). Appearances can be deceiving: instructor fluency increases perceptions of learning without increasing actual learning. Psychonomic Bulletin and Review DOI: 10.3758/s13423-013-0442-z