I built a white concrete map using commands but I typed the number one less than what it needed to be. I placed the blocks after I pick-blocked the white concrete (so it was definitely the same block) but the map makes it look like smooth quartz or some brighter white block.
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That's how maps work; it's a bit difficult to explain but TL;DR maps show elevation. if the block north of a block is lower, it will be rendered brighter, if the block north of a block is higher, it will be rendered darker. For more info, see https://minecraft.gamepedia.com/Map_item_format
What are the edges of the map like?
If there is a height difference in the Z direction (North or South) anywhere on the map including the edges, the map will lighten or darken the blocks next to the ledge.