Do you mean change systematically, within the same lead, over time? The slow visible fluctuation you can see in most people at the speed of respiration?

I'm guessing this is what you mean... it's the physical change in the heart's electrical axis over time, which appears as an amplitude change to a single dipole comparison. Please let me know if I'm not answering the right question.

I try to be simple in explanation.

In ECG, any lead showing upward positive deflection is due to depolarization waves coming towards that lead. QRS complex represents the electrical activity of ventricles and so its configuration changes in different leads are as below:

In V1 and V2 (located near right side of heart away from left side and mainly reflect right ventricular activity) the main QRS complex is negative because depolarization wave going away from V1 and V2 is larger than depolarization wave coming towards leads [ right ventricular mass hence depolarization is larger than left side]

In V3 and V4 (which reflect activity of both ventricles including interventricular septum) the main QRS complex is biphasic as now depolarizing wave coming and going away from leads are almost equal.

In V5 and V6 (present on left side of heart which reflect left ventricular activity) mainly the main QRS complex is positive as depolarizing wave coming towards leads is greater than depolarizing wave going away from leads.

[R wave gradually increases in size from V1 to V6 leads. In leads V1, R wave represents activity of right ventricle and in V6 of left ventricle.

S wave gradually decreases in size from lead V1 to V6. In lead V1, S wave represents activity of left ventricle and in lead V6 of right ventricle.]

What I am wondering is why in any single lead are QRS complexes not always the same height eventhough they are the same morphologically?

My guess is that there's a slight variation in the number of sodium channels involved for each event of QRS complex.Hope that makes sense.