Why would data transferred through an extension cable be different
Modern serial links are very complicated designs, and use many sophisticated protocols to adapt to "RF channel properties". They support various data rates and formats. Usually the link starts at minimal supported link speed, and performs "link training" by sending a stream of test packets. The receiver passes them in parallel through bandwidth equalizers (tunable filters), and selects the best filter that produces least errors, and then uses these optimal parameters. These programmable filters do equalization of attenuation of the channel, and restore signals to good extent. 

Then, typically, the link requests "capabilities" of link partner. If the partner says it can support higher speeds, the link switches into higher gear. Then it starts link training again at this new speed. If the training at higher speed fails, the link will fall back to known good speed. At least this is what happens in USB 3.2 between Gear_1 and Gear_2 speeds.  

So this is why you have bad screen resolutions if you add a bad quality or long cable into the channel.