Why another program may show a different chart
There is no agreed database of the world's old clocks. Every program that casts historical charts has had to research them, and they have researched different things, so two programs can disagree on the same birth and both be working honestly.
Where Arcus has no rule for a place and a date, it means nobody has established the fact yet, not that there was nothing to record. If your own source settles one, send it to support@arcusastrology.com with the place and the date and we will look at it.
If you know your source is right and Arcus reads it differently, set the offset yourself.
Didn't answer it? Write to support@arcusastrology.com.