2560 x 1440 vs 2560 x 1440 (low resolution)

I recently installed a Sapphire Radeon Pulse RX 580 8GB in my Mac Pro 5,1 and bought a 4K monitor. The default resolution for the monitor (3840 x 2160) makes everything too small. So I found that the 2560 x 1440 resolution works best for me. Interestingly, when I click Resolution Scaled in Display System Preferences, I'm only offered "2560 x 1440 (low resolution)". But when I option-click Resolution Scaled, I'm also offered "2560 x 1440". Doing screen shots for the two options, then enlarging the screen shots clearly shows "2560 x 1440" is crisper than "2560 x 1440 (low resolution)". Here's my question. Why would the better "2560 x 1440" resolution only be available with the option-click? Why is the low resolution the default? Thanks.