Cache Levels & The Histogram Panel
If you ever see a warning icon in the upper right side of the Histogram panel, it means that you are not looking at the current image data in the histogram. Click the icon to clear the image cache and update the histogram.
By default, Photoshop uses cached image data to generate the Histogram. To most accurately display the current data information on the Histogram Panel, Press Ctrl K (Mac: Command K) to bring up the preferences window. Under Performance change the Cache Level to 1, and then restart Photoshop.