Problem solved! Indeed it was because the model was in inches. I changed the units and the problem was no more.
Correct me here: the drawing says "Draw 1/4 size". Should that have alerted me to use milimeters or simply the fact that I was asked for an ISO hole? Usually these diagrams have "METRIC" written in front of them and that's when I comply. Since I'm just starting out it's natural for me to lose many hints or details.
Yes, these diagrams are lovely. I'm going through 140 of them before I start a mechanical engineering course later this year.