Because some RC60's are EC meaning they don't have a working FPU. I think all RC66's & RC75's are EC and have no working FPU. They may have FPU in them, and some may have an FPU which works at 75MHz but the EC means they were sold as not having a FPU. Some are LC meaning no FPU or MMU. It is a minefield.
You can get an MC68060RC50 E41J for $90 if you wait a short time. Plus the peace of mind that it has both an FPU and MMU and clocks to 60MHz without heatsink or fan.
I was about to buy some 060 66Mhz CPUs from a guy in the USA on ebay until I checked the revision and mask, seems like no 66Mhz 060 were made with FPUs and most have no MMUs either...
There's a neat site where you can find all the specs & details about all the 68K series of processors but I can't get at it right now as I'm on my old G4 and my G5 (where the link is) is currently installing OSX on a new HD that I've just installed, I'll put it here later when the G5 back up and running as it's very handy if your looking for full info if buying 060 chips...
