HI Kieren,
Mac OS is unix based, so it is likely to use standard unix system calls and library calls. On some unix systems, the temp file name generation algorithm includes a time based component. Maybe the tmp name changes after some time period perhaps? This is pure speculation, but could be easily confirmed by watching the temp file name over time.
I'll dust of my Mac and have a look for you - you can't be the only one, that would be strange.