This is what I speculate to be the case, I don't have any solid proof but I did some monster damage tests a long time ago and found that the damage of what should be pure specialized damage was less on Ancient Plate Mail when in Strata 5, and it also seems far easier to wear the right equipment (even taking into account the inherit damage resistances on lower difficulties) on easier difficulties, most likely due to the prevalence of normal damage.
As you get deeper in the clockworks, the many attacks gradually shift from being pure normal damage, to pure specialized.
This ratio is further modified by the type of attack (I presume most melee attacks have something roughly around double the normal:specialized ratio than ranged attacks) and the difficulty also applies a hefty modifier.
Let's just say for example, we have three depths: 19, 20 and 21.
Say a retrode's beam attack, on those depths, deals 80:20, 82:18, and 84:16 specialized to normal damage, respectively, on Advanced.
On Elite, they might deal something like 90:10, 92:8, and 94:6 specialized to normal damage.
(similarly, the Retrode's melee attack might do something like 40:60, 41:59, and 42:58 on those depths on Advanced, and 45:55, 46:54, 47:53 on Elite)
You can see this occuring in Tier 1 on Elite, monsters start delivering specialized damage well before when they would on Advanced.
I have never seen a detailed test or documentation of this. But if you believe what players say on the forums, Elite mode causes a lot of normal damage to be replaced with non-normal damage. For example, a construct that deals a mixture of normal and elemental will instead deal more elemental.
If you could find more detail, then that would be interesting.