Search results template not being used consistently

I have created an Archive Layout template. I set the Assignment Conditions to “Search Results” “is being viewed” and the “Priority” is 0.

I have a Search element in my site header.

When a visitor searches using that Search element, sometimes the results take them to a page using the Search Results template, and sometimes it doesn’t. It seems to depend on the search term!

I do not have any other Archive templates tied to the Search Results condition.

Why would this happen / how can I fix it?