A detailed explanation of the logic used in the Mosaic Method is defined in
Understanding the mosaicking rules for a mosaic dataset—Help | ArcGIS for Desktop
Specifically check the section on 'Rule orders'
The issue you are having I believe is related to CellSize Tolerance.
The default value is 1.8, but you may find you need to increase the value and then do a recompute cellsizes on the derived mosaic dataset.
What I think is happening here is that you have imagery that has different resolutions.Part of the ordering algorithm tries to ensure that only imagery that is not fuzzy is shown. Part of this process is to group together imagery (into levels) that has similar resolutions and select imagery that is in the appropriate level. The levels computed can be seen by right clicking on the MosaicDataset in the Legend and selecting Open, Levels Table. You will see that the imagery is split into level defined by Min and Max Cellsizes. You want to ensure that all the imagery that has similar resolution is in the same level. If you increase the CellSize Tolerance and recompute cellsizes you will see that the size of these groups increase. Its best not to go too fast up with the values, else multiple levels of the overviews end up in the same level which it not optimal.
Also note that the mosaic dataset property defined on the mosaic dataset it the default used by the mosaic dataset and image services. The actual method used is defined by the client application such as in the Layer properties in Desktop or through properties of the layer in a WebMap.