Race Predictor changing while resting

it could be that during your last run your vo2 max changed (decreased in your case) hence the adjustment. i noticed that my predictions fluctuate a bit as well - either way, i'll take those values with a grain of salt :-) - it's just a preduction and nothing more. also - you're being presented with a whole number but there's also an unseen comma :-) so in your case it could have decreased from eg. 50.4 to 50.1, still show you a vo2max = 50 and alter the predictions. https://support.garmin.com/en-IN/?faq=HUB4yrzJkg1BbgmozWkBm7 

Hi, yes most definitely. My last run was an easy 15k and usually my performance is always reported as going down during a long run... Not sure why!

well - perhaps in your case the easy run was a bit too easy - hr in very low z1 or despite the pace being low, your hr rose over what the watch learned about you until today and flagged the run; it could also be related to not being well-rested or under-slept (hrv related) - so there could be several reasons for the behavior... 

you changed your weight a little bit?

Same weight. 

The VO2Max went a bit down because Performance Condition went from +3 to -6 (end of the activity). Not sure why is that though. I felt perfectly fine and not tired at all.

What I was trying to explain here is that the day after (Today) the race predictor is showing slower times than yesterday after my 15k (when the predicted times went already down).

In other words, I would expect to see no variations during my resting days (2 every week). Hence my curiosity...

well - assuming you have your zones set correctly, 70% in z3 is a moderate effort (at least from a hr perspective), definitely not an easy one (although a trained individual could rate even a z3 run as an easy run); most probably about 20-30 minutes into the end of the run your hrv started to be fluctuate - aka wasn't in the known pattern hence the firstbeat algorithm marked the run as unproductive and lowered your vo2 max. as you have a somehow big volume to what your watch is used to, it detected the hrv changes - and concluded that you're tired, you should rest a bit more or take it really easy with the exercise :-) 

with this calculator you can see, wht happens, when your real weight changed a little bit and of course, mostly the vo2max is the same for me as with garmin:

https://thesecretofrunning.com/calculator/

fill in your age, female/male, finish time

Thanks for your detailed answer tsofron and I totally agree. My general fitness level keeps improving and only few months ago I couldn't really run in Z2 at all... Now I can but my HR tends to fluctuate easily from Z2 to Z3 after more than 30/40min and/or if I run faster than what I'm supposed to when running in "easy days". I'm not too worried about that though. HR is affected by a multitude of others factors and not only my fitness so usually I run "by isntinct" when in Easy workouts (but with an eye on my HR just to be sure). Btw, I use a Polar H10 so my readings are as reliable as it gets, I guess.

I'm training for my first Marathon and definitely trying to focus more and make sure my easy days are easy (I used to run my long runs in Z3/Z4 or even Z5 at the end of them...). I did a lot of research / reading and realized I was pushing too much. Since then I've been very strict about easy workouts being easy and hard ones hard! ;-)

Thanks