Drained oil of 1.4 litres sounds about perfect.

But, did you own the car from new? Perhaps the previous owner...?

Is your spare tyre a different size from your rear tyre size?

My spare is 225/45/17 and my stock rears are 245/40/17. The spare rotates at 807.6 revoultions per mile and the stock rotates at 815.97 revolutions per mile. Now you wouldn't think that would make much difference. But, my owners manual and decals plastered on my car WARN me not to exceed 80 km/h with the spare on the rear axle (BTW, the spare is identical to the stock fronts).

Perhaps you had different inflation pressures on the stock rears due to a slow leak at some time when you were driving fast for a sustained period? (Blame your wife LOL!).

PS. I just drained the diff on my C36 (157,000 km) about two weeks back. I am not sure if it has ever been changed since new. It smelt great and the oil looked really good. So if your diff oil smelt burnt, it probably was burnt.