I really doubt you were able to burn 10x CD-RW media at 24x. CD-RW media is not as forgiving about burn speed as CD-R media, and as far as I know no 10x CD-RW media can be burned at 24x. They just aren't physically capable of being burned at such speeds.
I suspect that what you are seeing now is normal, and has been the case all along. Perhaps you burned something else at 24x and confused which media burned at what speed? Or maybe you had an older version of Nero that incorrectly allowed you to select 24x, but actually burned at 10x or 12x?