It would appear that someone has replaced one of them in the past. The metric actual axles are only a couple of mm's shorter than a imperial. So you will find that an metric axle will fit fine in an imperial diff, but not necessarily the other way around. The metric drive flange bolts are thicker than the imperial ones. So you will find the imperial bolts will allow a metric axle to be fitted, but not the other way around. But this would mean a sloppy fit for the metric axle.