The default frame format depth option applies to creator tools like Background, FastNoise etc. In Loader preferences you can specify whether you want it to load files at the native depth reported by the file format reader (the 'Format' option) or whether you want it to be the default frame format depth (the 'Default' option). What that Loader preferences option does is determine the initial value for the Depth option in Loader's Import tab. You can then use that option to override/change that initial value, similar to the Image tab of other creator tools.
The depth value that the view shows is whatever the image you are viewing actually is.