Teacher learning in asynchronous online professional development (PD) is often assessed through self-reported instruments, despite uncertainties regarding teachers' ability to accurately self-assess their learning gains in such settings. This study compared middle school mathematics teachers' self-perceived and observed learning gains in content knowledge (CK) and pedagogical content knowledge (PCK) after participating in an asynchronous online PD program. By analyzing data collected from 57 teachers, we found a weak and nonsignificant correlation between teachers' perceived learning gains and observed learning gains. Teachers who frequently monitored their understanding of the learning content tended to underestimate their improvement of CK from the PD program while those using cognitive learning strategies (i.e., organize and elaborate) often overestimated it, controlling for their baseline knowledge level. These findings highlight methodological concerns in using self-reports to evaluate online learning outcomes and the potential distortion in identifying effective learning strategies.