Going out on a limb here.
BUt the RCD measures Leakage to Earth, as per the above Current measurement comparison, if its not the same, it is leaking back through the earth.
As lights generally did not have an earth leakage, im presuming the theory is that it cannot leak back to earth, and therefor didnt go through the earth leakage.
Im, pretty sure some of this has changed in the regulations, specifically with newer lights and LED's having an earth.
I recall having a "light" issue when I bought my current property, replaced a bunch of bulbs with LED's some of them would turn on very dim when the light switch was off. Measured with a voltmeter, and there was enough power leaking to create a 30V circuit that turned them on when live power was disconnected.
My lights were not running through the earth leakage( Since rectified), but turned out to be a faulty day night switch creating the leak.