Maybe 20+ failing retries for any single page build?
Guessing absolute failure rate previously not encountered by current graphing tool, incorrect scaling factor. Or, it was decided at some point that always reporting 0.000XXX% failure rate, even if correct, didn't offer an intuitive metric, so zeroes were intentionally truncated.
It'd be nice to have an intuitive explanation on the status page for what PBFR means if it can go over 100%.