180 SPM is considered the most efficient. What device are you using to measure your cadence? Is it taking an average of your entire run or just the fast sections?
For the next time you do this workout, focus on taking shorter strides but pushing off more with each "mini-jump" during the 1 minute at 6min/mile.
I run with a Garmin vivoactive hr - and it was the average for the run. My fastest cadence was 206 spm during one of the fast sprints.
I will work on my push off on next week's fartlek
Wow! 206! Try to bring it back down to 180 during the fast sections then. That should help you to conserve energy more efficiently.