That could have something to do with the discrepancy between what you are seeing and what was observed on our review sample. For this review
we were using an AMD setup. Colours appear slightly richer and more vivid on modern AMD GPUs (all else being equal) and this certainly reflects feedback we have received from Nvidia users who have ventured into things like 'Digital Vibrancy Control' (DVC) settings to try to add a bit more 'pop' to the XL2420T's image. That could certainly be worth a shot if all else fails, but don't overdo it as all you do is stretch out and oversaturate colours and reduce shade variety by going too heavy on the DVC.
Before going there it is worth bearing in mind that Gamma Modes '4' and '5' produce an average gamma of around 2.4-2.6. This means dark shades appear darker, richer and more saturated than they should whilst light shades appear lighter (and more washed out you might say). In addition to this the BenQ XL2420T adjusts the tone response at the low end even in standard mode. In other words it effectively applies the 'Black eQualizer' to some degree even in modes where it appears greyed out (including Standard Mode). This lightens dark areas - coupled with the gamma settings this could be causing quite some reduction in shade distinction. These concepts are explored in the review and so is the idea that the monitor has a lot of settings to play with and you shouldn't be afraid to experiment.
TLDR; don't take the settings as gospel. Nvidia and AMD GPUs output the image slightly differently and each individual BenQ is different. Experiment with different gamma modes, keep the colour balance at default and reduce the channels individually if you feel the need and perhaps even experiment with Digital Vibrancy Control.