Garmin's race predictions

I've been working toward breaking the 20-minute barrier in the 5K. For the past few months, my Garmin (Forerunner 255) has consistently predicted times between 20:10 and 20:30, which made me uncomfortable trying for a sub 20.

Yesterday I was feeling good, so I decided to ignore the prediction and just go for it, even though Garmin estimated I'd run a 20:18. I ended up finishing in 19:20!

Obviously happy with the result, but it also surprised me how far off Garmin's prediction was. I've been using this watch for the past two years and have logged plenty of tempo and interval sessions with it, most of them while wearing an external heart rate monitor.

Has anyone else found Garmin's race predictions to be this inaccurate, or is it just me?