Many factors can affect this... your laptop graphics outputs are optimized to work more efficiently with your graphics card, which i'm assuming is separate and not shared with the mobo for graphics rendering.
There are so many questions that affect the output, example a usb hub that has hdmi output(S) has a chip in it that is used to convert the signal back into the HDMI configuration from the serial data. Pushing 4k and HD gaming may overwhelm that chip as it was probably designed normal computing multiscreen displaying.
To track the problem down you will need to invest in a True HD capture card so that you can evaluate the raw HDMI signal.
If you are gaming, it is cheaper and less time intensive to just utilize the HDMI output of the computer to a HDMI hub designed and optimized for exactly what you are wanting to do (which is to extend your screen i'm assuming), which is run multiple HDMI devices (If the displays are not identical and capable at the same refresh rate and res, (you will see differences as well). In your computer Your computer has to take the raw video signal and compress / convert it for USB. Then your usb hub has to convert the compressed signal back to HDMI configuration via its onboard chip, and then onto your HDMI displays.
To test this reduce shading and resolution down to a normal computing level, observe if the displays now look the same. But first balance the black levels and brightness between the displays by independently, one at a time sending a signal to each at the lower shading and res. Once displays have a balanced black level and brightness... perform the test just mentioned.
My guess though is that if you have a monster computer meant for gaming that the problem is not there but in exceeding the video conversion capabilities of your hub. cheapest solution is to just leave the outputs from the computer to the two displays split and live with it. If you can't, seek a hardware configuration meant for this endeavor.
The only way to properly diagnose your system and configuration is with detailed connection layout, specs on all components in the configuration, and then lengthy testing.