[flatheadmurre]

The bimetall flashers are simple creatures.
When you apply load to it a bimetall is heated upp until it opens the points...cools off and closes again.
The higher load (w) you apply the more heat it manages to absorb before opening...that´s why full load flashes slower then just one bulb.
The pilot light is just another set of points working in parallel.
I prefer 21w bulbs over the 18w...only difference is perhaps you can notice a bit slower flashing.