Slower workouts

Today on the cross-trainer I had my slowest longer workout since September 2018. I used to aim for 25 of the cross-trainer's calories per minute then, in slacking off, I figured it suffices if I exceed 20; today I hit 19.2. I am okay with that, more curious than anything. I put the time in on the machine, I sweat, elevate my heart rate, I figure I am doing the right thing; I plan to shake up my workout routine next year anyway. Still, I find the regress curious.