0%/100% cacheability for single-use layouts

When running Hugo with --templateMetrics I am getting weird lists like the following:

        100     252.326µs     252.326µs     252.326µs      1  partials/icons/linkedin.html
        100     250.927µs     250.927µs     250.927µs      1  partials/head/opensearch.html
        100     242.778µs     242.778µs     242.778µs      1  partials/icons/twitter.html
          0      189.71µs      189.71µs      189.71µs      1  _default/index.security.txt
          0     171.164µs     171.164µs     171.164µs      1  _default/index.redir

The first three items say100% cacheability while being used only once and the last two say 0% cacheability. All layout files are from within modules, not the Hugo running repository itself.

Is this a bug or is there some deeper logic behind it? Maybe that caching might take more time than parsing these single-use files without checking for cache? The only difference I see is that the 100%ers are all above 200 microseconds and the 0%ers below 200.