My 2009 printing of the Modern Reloading Lee 2nd Edition doesn't list the 460 S&W so you must have a newer edition.

One problem with the Lee manual is they don't give a lot of necessary data to evaluate their load recommendation. They only list a Minimum OAL, for all the good that does, and not the actual OAL of the tested load. They also don't seem to mention test barrel type or length, case or primer used. Notice also that pressures are listed in CUP, PSI and N/A. I'm pretty sure they don't pressure test anything and just use whatever is listed in their original data source, but they don't list their source of the data.

I have heard that the Lee book is just a compilation of various manufacturer load data, collected in one volume but with a lot of necessary data left out for formatting sake to include their load disk info.
Sure that book is useful if you use Lee powder measuring equipment, but for all other uses, for me, it's just a cursory additional source to verify other load data sources. In other words, I take that book with a grain of salt unless it agrees with what I have found elsewhere.

I have the Lyman #48 so once again I can't look at the 460 S&W data. Comparing the two, the Lyman book appears to be much more detailed in any given info they provide and I would tend to believe Lyman more if I had to make a choice,however they too can be way off from other reputable sources.

When I see a big difference in data, the first things I check are the OAL and the shank length of each bullet they used for their load. You could always load up 3 at the lowest starting load and then increment your way up.

One thing that looks suspicious is that you listed the exact same velocity for both loads... is that correct or a typo? If correct then one of those loads is highly suspect.