In some raster formats their can be georeferencing information built into the header of the image. This information can override the world file information.
If your tiff is a geotiff, this could be what is happening.
ESRI help offers the following advice:
Set ArcGIS to read the world file first
ArcGIS will automatically read the georeferencing in the header information of some file formats, such as GeoTIFF, which will override any georeferencing information stored in a world file. You can change this by checking the Use world file to define the coordinates of the raster check box on the Options dialog box.
- Click Customize on the Main menu and click ArcMap Options (or ArcCatalog Options, depending on the application you are using).
- Click the Raster tab in the Options window.
- Within the Raster tab, click Raster Dataset and check the Use world file to define the coordinates of the raster check box.
- Click OK.
Source: ArcGIS Help 10.1
Chris Donohue, GISP