After a bit of digging and fiddling, I have the following additional info:
The drill file's coordinate precision was 3.5. The problem goes away when I reduce the file precision to 3.4.
So, it appears that gerbv cannot handle more than 4 digits after the decimal place.
I also find it strange that, with a file precision of 3.3, the autodetect doesn't pick up the right number of digits and I have to edit the file format after importing.
After a bit of digging and fiddling, I have the following additional info:
The drill file's coordinate precision was 3.5. The problem goes away when I reduce the file precision to 3.4.
So, it appears that gerbv cannot handle more than 4 digits after the decimal place.
I also find it strange that, with a file precision of 3.3, the autodetect doesn't pick up the right number of digits and I have to edit the file format after importing.