Sapphire seems to sell two versions of their Nitro+ RX 590. One will boost to the common advertised speeds of 1560/2100 MHz and one version won't tho they are named exactly the
Sapphire Radeon NITRO+ RX 590, 8GB GDDR5 | CZC.cz
SAPPHIRE NITRO+ Radeon RX 590 8GB GDDR5 Special Edition