It is probably that their darks are more dark… but I could be wrong.

Show some examples.

It’s very rare that white objects are actually white. In fact, due to reflected colour, white really isn’t white. So, it becomes a matter of what those reflected colours are, the light source and values.

My tutorial here on WC demonstrates how to paint a wild white rose, When White Isn’t White.

Here are some examples of my approach to white that are not in my document above:

here is one… see that white fence and white table… they arent white but they looks white.

here is some sargent’s painting, look at that white building and white clothes.. they arent white.. but they looks white.

The table has a dark patch of purple/magenta just near the part that looks most white. The underneath of the fence is also inappropriately darkened (the lawn behind should not make sudden jumps in color and values).

The white building has shadows and darks all over. The building is also the brightest thing on the paper. Despite that, a more colorful part of the building looks brighter, the floor near the dark mass of boats and people, and the wall adjacent to it in the same reddish color (which we assume is sunset, since we assume that the building is white).

The sharpest and apparently darkest shadows on the clothes are adjacent to the brightest areas. One also has to remember that it works both ways for the human eye. Dark will also look darker when placed next to a bright area.

Eyes will automatically perceive the brightest areas as white (within reason), if there is no other whiter object (like the mat and frame) in the field of view. This is rather similar to how a digital camera sets white balance in auto mode.