Fault finding job

[Calebp43]

Did a fuseboard change yesterday from an old wylex board to dual rcd on a TT system with 14ohms ZE and finished pretty late. Went back this morning to test and was told that her cooker was tripping the rcd.
Before checking the cooker element I did an rcd test this morning on the left side of board (side with cooker circuit) and the rcd wouldn't trip. Did an rcd test on the right rcd and it would trip both rcds. Would touch neutral to earth on the cooker circuit and nothing would trip.
Anyway the fault was that a neutral was touching the outer casing of a light (see photo) that makes sense about the rcd tests, because the fault current the rcd sends out will go through the earth and back up the neutral cable of the lights and make its way back to the rcd.
I am a bit confused though on why this neutral to earth fault didn't just immediately trip the rcd? Am I right in thinking that it's probably due to the highish ze of the TT rod? So the fault current just wants to flow through the neutral as appose to the earth and it would only trip with the cooker because enough current is drawn therefore more fault current and more current going through the earth as well as the neutral