Problem with saving the file.

If you used Save For Web it does not set any resolution setting in the saved file  data for resolution is meaningless on the web. Displays can not change their resolution.   Photoshop need to have some Resolution setting to be able toe edit a document.  If Photoshop opens an Image file that does not have a set resolution .  Photoshop will use its default 72ppi.  The document will still have the pixels you saved it will just print a little  larger then your original 75ppi resolution.  You cans easily change the resolution to 75 ppi use image size un check resample and set resolution to 75 ppi.    Use Save as to save resolution and meta data  Save for web default is to strip  meta data..