That could have something to do with it, but as someone who lived in WA for quite some time - you don't go outside without clothing for ~7 months in any given year in WA either because it's raining and unpleasant. So that's where this hypothesis falls apart a little. My hypothesis is that constant relatively low level exposure to UV is more beneficial than acute exposure for just a few days / weeks in a given year. I don't have any data to support this hypothesis though.