It may be sorted according to your preferences or settings in the Styles pane. Click on the dialog drop down (bottom right of Styles Group on Home tab):


and then on Options.


You get this:




I suspect that if you change these options to All Styles and Alphabetical it will change how the list is displayed in the Find and Replace dialog.

Thank you for pointing that out, Charles. In the options, "Select how list is sorted:" was set to "As recommended". I don't remember doing that. Anyway, I set it to "Alphabetical" and all is well now.