I had a question about differences in conditions during a particular swell -

Some days, it seems that the ocean is firing - waves are constant, the paddle out can be really difficult and the currents moving around the water are very strong. Other days, you can paddle out without getting your hair wet, the water is very calm, and the waves primarily in sets every couple of minutes or so.

The waves and forecast (winds, size, etc) during both of these days are the same - however, the conditions vary greatly. This happens quite often, for example a week or two ago during the Leslie swell.

What causes this variation? Is there another variable that I am missing? Just curious for my own education.