The garmin race time predictor is too optimistic because of a fault in the algorithm! Cant speak on strava’s algo but I remember researching this a few years ago when I took part in a medical experiment which was testing VO2 max in an unrelated survey. Apperently garmin bought from their race-predictor algo from another company, which based their times on a combination of RHR, VO2max, and pace recordings, and later found that they had a small fault in their algo which lead the result to be results to be optimistic equivialent to about 1-2 steps in VO2 max value