When working with the dot density symbology method, the symbology dialog provides controls that preview the min, mean, and max dot densities on the map. These controls help you gauge the proper dot value and dot size. You can, however, encounter a situation where these controls incorrectly represent what is happening on the map.
Changing the coordinate system of your dataframe (changing the projection of your map) will cause any existing dot density symbology to have incorrect previews.
Dot density symbology is initially built based on the data frame's coordinate system. The properties of the data in this state are used to determine the min, mean, and max densities for the dialog box. If you subsequently change your map to use another coordinate system, the preview densities will not update correctly.
After changing the projection of your dataframe, to correct the preview dot densities, remove and reselect the value fields on the symbology tab of the layer properties dialog box. The preview densities will be correctly recalculated.