Hi - alpha pots have 20% tollerance so yeah 470k, 500k not a lot of difference, though obviously a 470K pot is going to have a range of 423k to 517, where as 500k are going to have a range of 450k to 550k which might make a difference depending on the maximum and minimum resistance values required.
I have tested them before and found that the range has been well within 20%, but I guess had I tested several hundred rather than a handful then there would be the odd one that was 20% out.
These pots can be manufactured in increments of 1K (not sure how Rapid manage to get 470R), so 471K for instance would pointless because if you needed it to be that precise then you would be wasting your time with carbon track pots, but it does give me the option such as in this case whether I want them in multiples of 5 or 0.5, or 47 or 4.7 etc.
The reason I stock 500K instead of 470K, or 50K instead of 47K is because they are the values that people ask me for, either when they are replacing a pot in a piece of hardware or sometimes when they are following schematics which I guess are from US websites or books. It just makes sense for me to sell them in these multiples.