[GAMEPLAY] Demolish rune sometimes deals little to no extra damage

I noticed in a normal game that Demolish dealt little to no bonus damage after waiting for initial 6 ticks, especially if I use gargoyle stoneplate.

Problem: Demolish rune sometimes deals little to no extra damage, also when coupled with gargoyle's stoneplate

To verify, I tested this in the practice tool. Here is what I did:

1. Picked Sion or Cho'gath (tried both)
2. added levels to level 11 (if Cho'gath, i got 6 feast stacks)
3. added 40k gold
4. bought ninja tabis, warmogs, gaygoyles stoneplate, omens, sunfire cape
5. tp to turret, charge up the demolish rune with or without friendly/enemy minions (tried both)
6. attack, and always deal around 150-300 damage total if I use gargoyle's stoneplate, as Sion. as Cho'gath, i dealt around 500-650, varying damage
7. if i do not use gargoyle's stoneplate, I deal ~~~800~~ varying damage to the turret as Sion. as Cho'gath, I dealt around 1100varying damage, sometimes 150, sometimes >1000.

Conclusion: I think it has to do with hp totals exceeding a certain point, then getting reset when calculating for demolish damage. So 6000 hp might count as 1000 hp, while 5000 hp counts as 5000 hp. Just a guess though.

Cho'gath hit without gargoyles, level 11, first 3 mins of game, tp under turret, no friendly minions pic1

Cho'gath mid turret: hp before hit w/gargoyles (hp lowered because of minion wave) pic2

After hit w/gargoyles pic3