Frankly, whoever thought that the same action (open) on the same type of file (e.g., JPEG) should be inconsistent needs to re-evaluate his/her ability to apply logic to a given situation.

The solution seems to be to find a file that is opening in Photoshop and should be opening in Preview, and then open its “Get Info” dialog (right-click, Get Info or command+I). The fourth section down is “Open with” (you may have to click it to expand that section). Change the dropdown to Preview and then click the “Change All” button. That should fix the problem….

…for that file type. Yes, you seriously have to do this for each file type (JPEG, PNG, GIF, etc.). So, after you perform this same sequence of steps several times, you’ll eventually be able to have consistent, expected behavior. That is, until you use someone else’s computer.

If anyone knows of an easier way to fix this problem, please let me know. Otherwise, feel free to use the comments for general grumbling about the issue.