Energy Consumption of AC Motors with same capacity and same hp
You need to make a few more measurements, as your assumptions (same motor type, same load magnitude, and the implicit same run duration) are not borne out by your observations.

Before it's worth swapping the motors, you need to get to the point where your observations are repeatable. In some cases, for same load size it is vice-versa, motor 2 consumes 7kWh and motor 1 consumes 4kWh. If by this you do really mean that the relative motor consumption varies by a factor of 2 without explanation, then you don't have a hope of doing controlled experiments with your present assumptions.

A few things to try. Are the ingredients in one line aerated or more diluted, and so easier to move? What about temperature? Are the source and delivery hoppers sometimes at different fill levels? Does one line stop and start without you noticing? You know your plant better than we do, crank up your imagination. Is there another load on the consumption meter that sometimes draws power? Start guessing what factors could affect it, start measuring them, and see if you can find a reason for the apparently unexplained variability.