Is lower average body temperatures.

The general recommendations that you grew up with, that 98.6 is a baseline average? That was based on research done in the 1800s, back when people lived a generally more active lifestyle and when healthcare and diet were of poorer quality.

https://www.healthline.com/health-news/forget-98-6-humans-now-have-lower-body-temperature-on-average-heres-why

So yeah... If your temperature is 'average' or slightly above, then you may in fact be running a fever and should quarantine yourself.