I'm confused there's some(all) golden gear are less than my gear that I have in blue for example

already know about enhancement the cap is + 15 if  have someone time can you teach me. or on Xbox the marketplace are out of whack.

You need to go and watch some YouTube tutorials on equipment enhancement and failstacking.

Why is the cap on 15? The cap on xbox is pen (20). Delphad can't be upgraded and kzarka can be upgraded to pen. If u wanna see how strong kzarka can be  why u don't press A on the kzarka and open the list for all kzarka levels? Where u made the Screenshot its the Liste before u can see the stats on every lvl. Cm - weapons- main hand - amulet-kzarka--+15kzarka.

Even on 15 the kzarka is much stronger than a delphad. 78-82 ap and 160 accuracy

Here's a good guide

https://grumpygreen.cricket/bdo-gear-progression-guide/

Also just a heads up...once you get gear to duo....if you try for anything higher and it fails it will go down a level.  Accessories are more of a challenge in that whenever they fail they explode and are gone.