I had almost forgotten about the Excel sheet I made. But you have a good question. The discrepancy in feed rates is driven mainly by two factors. The first is that the Router most people are using is just way to fast for most of the tools being used on the X-Carve. The high RPM is forcing the calculated feed-rate to be pretty high in order to generate the recommended chip load. But then the second factor kicks in with a vengeance. The X-Carve is a great machine, but it is not a super rigid CNC add that to the relative lower torque of the router and the X-Carve is just not physically capable of running at the faster feed-rates.
So as I have said many times on this forum (in the distant past) - Just make the best of the situation with the tools you have. Unfortunately this means you are not going to be able to generate optimal chip loads. So we reduce the feed-rate to what the X-Carve can deal with and reduce the RPM’s of the router as much as possible and compensate with the only other variable we can really control, the depth of cut.
And for the most part you should be able to get some great results from the X-Carve.
The main reason I made the spreadsheet was to help others realize just how fast the Router was spinning compared to where we needed it to be, I had many discussions with some very smart people on the board (years ago) about what the proper speed setting on the router should be. There was one camp that would just set it to 5 or 6 and leave it there, thinking faster is always better. I hoped that my spreadsheet would help show them that even at a setting of 1 the RPM’s were way to fast for most of the tools being used.
I think for the most part now that way of thinks has been pretty widely adopted.