gerbv is misreading the numeric format in an Altium Excellon drill file. The top of an example is here:

M48
;Layer_Color=9474304
;FILE_FORMAT=2:3
INCH,TZ
;TYPE=PLATED
T1F00S00C0.028
T2F00S00C0.035
T3F00S00C0.040
T4F00S00C0.047
T5F00S00C0.051
T6F00S00C0.072
T7F00S00C0.125
%
T01
X1875Y350
X1965Y460
etc.

Tracing execution in drill.c, it is correctly parsing the FILE_FORMAT=2:3 line, but the subsequent INCH,TZ line resets to trailing zero suppression with 4 decimals. The file then reads at 1/10 of the proper scale.

I don't know if Altium is at fault, but without a detailed knowledge of the standard, it looks incorrect to set TZ because the rest of file is obviously not suppressing trailing zeros (a dumb concept, but there you have it). But an Excellon guru needs to look at it.

If, in gerbv viewer, the format is manually reset to 3 decimals (no autodetect), it reads at correct scale.

Possibly, state->autod should be reset if the FILE_FORMAT line is read, or another flag should be set to prevent any change to the format.

This may be related to issue #94.