Many of the tropicals will also be food for the local fish before they die, once the water starts getting too cold for them, they stop eating and swim more slowly and become easy targets, especially given their colors. They are part of the natural food chain, so you can certainly interpret them being misplaced relative to how we think about it and where they "should" be. When the waters start to get real cold, and we get a cold snap, we have seen days where you'll find dead butterflies in shallows.
Some of the other fish that we view as tropicals do have migratory skills and head back down south as well.
As reefers we focus on the tropicals, but the same is true about the Northern fish that come to our waters in the winter time with the Labrador current. We live in a truly dynamic region.