It is because of the superimposition of wind seas with swells. During which the local winds (say, sea breeze) are stronger than synoptic winds, hence wind seas are higher than swells. We know that swell periods are higher (say, between 8 to 12 s) than wind sea periods (3 to 5 s). when they superimpose, the mean wave period decreases (bewteen 5 to 8), however due to strong local winds the wave height increases.
Please refer the following paper, for more information:
Vethamony, P., Aboobacker, V.M., Menon, H.B., Ashok Kumar, K. and Cavaleri, L., 2011: Superimposition of wind seas on pre-existing swells off Goa coast, Journal of Marine Systems, 87, 47-54.
Dr. Aboobacker V.M.
Tropical Marine Science Institute (TMSI)
National University of Singapore (NUS)