Chances are that you saw the in-store demo playing some UHD media file clips rather than real world content. The TV might also have been set to demo mode. This ramps up the contrast to make the picture stand out as much as possible against competing TVs.
As for shop staff adjusting the picture, this rarely happens. Very few know what they're doing, for a start, and there's rarely a decent source player and test disc to use because the staff wouldn't know what to do with it. The other reason is that, when correctly adjusted, the set won't look anything like as bright and dynamic as its neighbouring competitors. That means then they'd all have to be adjusted. Shops stick the TV on demo mode if it doesn't already default to it out of the box.
When you got the TV home, you may have had a choice between Home or Demo mode during the set-up process. After that, you were left to make your own choices from the various picture modes. None of the presets get the picture right. That's because the TV set-up depends very heavily on the lighting conditions in your room and, to some degree, your sources too.
Some of the settings (backlight, brightness, contrast, colour, sharpness) are best set with test patterns. The room lighting levels affect some of the foundation settings, and they then in turn affect the settings that sit on top such as colour. This is a large part of why using someone else's TV settings doesn't really work: Every room has different lighting compared to yours, but also, your own room experiences a huge change in light levels from say a bright day through to night time viewing. How do you know then whether the other person's TV was set up for day time or night time viewing; and that's before you ask yourself if they actually knew what they were doing before they posted a set of numbers online.
There are some other controls on your set where the settings are more predictable.
In general, most of the 'picture improvement' settings such as Dynamic Contrast, Colour Enhancement, Noise Reduction, Motion Smoothing etc is a waste of time if you want a good picture rather than a plastic-looking mess. Turn it all off.
