Well Lower is a text function so it makes no sense to use it for sorting on a number or date field. Text based sorting IS case sensitive in SQL so using either Upper or Lower makes sense for text based sorting.
Or are you getting unexpected results when you sort a number field and you are not using the Lower function?
If so, make sure that you are really using a field of type number and not of type text to store your numbers. At least that's the most common reason numbers in FileMaker might not sort as expected.
Thanks, Phil. Dumb error on my part. You're right; removing LOWER fixes it.