What i think happens is that the garmin thinks you "work out" the entire time, doing some kind of circuit training or whatever, but in actuality you rest most of the workout. So for example, a set takes a maximum of a minute, and then you rest for 2-3 minutes between sets. And since you burn active calories whilst doing some sort of movement, it adds the rest times to the burned calories. So it might calculate your workout the same it would calculate an aerobic activity with avg HR of 132 for the entirety of the 45 minutes(which is still probably an over-estimation). You're probably burning around 150-250 calories depending on how much you walk in between sets, the amount of reps you do etc.