Gearing question for newbie

Gearing question please.

I did a couple of group rides over last couple of days, first one was a fair bit quicker than second; however to maintain the same watts in the second one I had to go to up gear 15/16 compared to 12/13 on the first quicker ride.

Why would this be? Both were same flat course, I did not change any settings yet if I used gear 13 I was approximately 40 watts down to previous day.

Very difficult to say.
There’s drafting which has a considerable effect.

What were the events?

shouldn’t impact the gearing you need for the same wattage at the same gradient.

what trainer/power source are you using? does it need calibrating and when did you last calibrate it?

It’s the Zwift hub one and I didn’t calibrate it when I got it. Do you need to calibrate them???

It should have autocalibration, you can force it by riding for a minute then getting to a speed above 15 km/h and then coasting for at least 5 seconds. It will recalibrate each time you do this:

if you notice it again maybe just coast for a bit and see if that fixes it.