What does this mean?

[Kmxxx14]

If I do the exact same workout on different days or even weeks apart - in the first workout I spend 4 mins in my peak heart rate zone and the majority of the time in cardio zone (according to Fitbit) and on the second exact same workout I spend 20 mins in my peak heart rate zone and the remainder in cardio.

Does that mean I pushed myself harder during the workout mostly in peak? Became fitter?

It might well vary with your cycle, as your hormones will fluctuate making workouts harder and easier across the month.