I am sure someone will come out with the exact reason for the sizes,
but it seems that the burs are made to the more common stone cutting
sizes. In a lot of our rings I use diamonds that are cut to 1.6mm and
1.8mm. That means I have to use a 1.5mm burr and carefully "enlarge"
the hole. Basically I cut the seat to depth, then rotate the bur in a
circular motion to make the hole bigger, does this make sense?
Another thing that will help, start the hole with a bud bur, which
will be slightly larger than the finished size, giving the opening a
little taper, cut the bottom seat, then rotate the bur. Remember
though, better to cut a little, then have to cut a little more, than
to cut away to much, so gently open the hole.
Hope this helps.