You're so not missing something. Since version 5.5 of FMP I am unable to get working cross-platform solutions without tremendous amount of work and work-arounds.
I don't know if this should be reported as a bug or if it's caused by the OS rather than FMP, but here goes:
Products affected: FileMaker Pro 10, FileMaker Advanced 10
Special note: I have copied the Verdana font from my Windows installation to the Mac OS X Fonts folders (and removed all previos instances, look out 'cause there are a lot of Font folders in OS X...)
Steps to recreate:
1. Take any FMP file, new or existing.
2. Go to layout mode.
3. Grab the text tool, click and type "This is the first line" (yes, you can type whatever you want! ;)
4. Format the text as Verdana, 10pt. Keep line spacing to "Single".
5. Click the text item and create a new line (hit return) and type "This is the second line"
6. Hit return again and type "This is the third line".
Verify that this text item looks exactly the same when viewed on Mac OS and Windows. Line spacing should be the same on both systems so that if you draw a box using the rectangle tool exactly around the text in OS X, the text shouldn't fall outside the box on Windows (or shrink).
You can now change the font to bold, or change the line spacing to double or a specified amount. Works fine. Text item should remain identical on both system.
Change the font size to 9pt and watch the layout fall apart. Line spacing is now way bigger in OS X than in Windows. Why, Oh why???
Also my testing shows that using any other font (any other font that exists in both systems that is) will not work at all. Only Verdana 10pt can be used for cross-platform layout as far as I know.