Nothing to do with speed, it estimates it from power and HR. Are you actually recording your TR sessions on a garmin device? As it won’t estimate VO2 from third party data. Mine tends to vary quite a bit based on all the factors that cause HR to vary (temperature, time of day, caffeine, fuelling, etc) but more in a range of maybe 5, never seen a drop anything like 10. Did you change your HR zones or weight on your garmin recently?
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