mtbc | Variation in workouts

In today's workout I performed over 6% better than on Thursday's otherwise-identical one which seems quite a difference to me. I have mentioned puzzlement over these differences and one of my suspicions was variance in how the cross-trainer sets the resistance level. I noticed that I was breathing rather harder today so I guess that the resistance is not the explanation: at least in this case perhaps the change is not in the machine but in how hard I feel able to work out. This does not get me much closer to predicting how well I will do next time. I try not to mind if I do worse on some days; I don't want my workouts to become an overly difficult chore.