So shortly I bought rx580 nitro 4gb version lately but it works on 1340mhz instead of 1411 also memory clock works on 1750 instead of 2000, so I'd like to know what's that and whys happening, thanks.
There were 4 SKU models of that card, 2 are listed as 1340/1750.
Check which SKU model you have on SapphireTech site.
Fairly sure only the 8GB cards were 2000MHz (the odd exception).