I'm pretty sure that that dialog calls MessageBox(), which just displays a string. One string. Any formatting that Rex does is done by inserting line feeds and tabs into the string. And how far right a tab takes you depends on what's to the left of it. If the text before the tab is unusually wide or unusually narrow, then the text to the right won't line up as intended. That's just the nature of the beast.
I imagine Rex could create a custom dialog, with individual text boxes for each field. Whether the more reliable formatting would be worth the time and the increased program size is a question of theology.
Windows isn't perfect when it comes to tabstops in variable pitch fonts. There are tabs (0x09) after "Last Write:" and Last Access:". With Charles's set-up (mine too) the string "Last Write:" ends right before a tabstop so the tab character does nothing.