S2719DGF, super weird phenomenon

So I have dual monitor setup:
Dell S2719DGF for primary gaming
Asus MZ27AQ for multimedia (youtube)

I saw some configuration online for Dell S2719DGF that would theoretically improve gamut output for color ranges (TBH I have no idea what I'm talking about)
and started to play around with my monitor settings and colors and such...

To test my black uniformity, I just searched up "black 4k wallpaper" and displayed a same image on both monitors
and they are shown different... I mean wayyy different

so this is the link to the image that I was looking at
Asus monitor showed this black
on my Dell monitor the black wasn't black.. I clearly see some red hue and black painting scribbled on it...
At first I thought I was going insane.. but the more I looked at it and thought about it, it seems that there is actually something like watermark on the black image that Asus monitor wasn't able to render.. and only seen in Dell monitor..

Now this questions my vision capability - cause I've enjoyed watching multimedia stuff on my Asus monitor way more than Dell monitor because I thought Asus monitor produced much deeper colors (at least to my eye)
but it seems that Dell monitor is actually more suitable for watching? because it can render more colors than Asus monitor can?

i just want to ask - is watermark on the link that i've posted expected to be seen on monitor?

Thanks in advance!