How it works
A bottleneck is the stage that holds up the work you want to complete. The processor may prepare simulation and drawing commands while the graphics card renders them. Crowded scenes, resolution and frame caps can change the limiting stage without changing any hardware.
Important differences and limits
A component-pairing calculator cannot know every scene, background process or cooling condition. Low overall CPU utilisation does not rule out a busy game thread. Slow storage or insufficient memory can also cause interruptions resembling a graphics limitation. More expensive parts do not guarantee a balanced result.
A useful practical check
Measure your usual game with frame times and processor and GPU activity visible. Lower rendering resolution while keeping the scene unchanged: an improvement suggests graphics work mattered, but is not a complete diagnosis. Check temperatures, memory pressure and background tasks before selecting an upgrade.
An example in practice
For example, a crowded strategy-game scene may demand more simulation work while a high-resolution scenic view demands more graphics work. Test both if both matter to you; a recommendation based only on the easier scene can misidentify the useful upgrade.