SA Post Office
Does overnight delivery not mean OVERNIGHT?

I posted a parcel via post office onernight speed services, Parcel was posted on Monday 11th February 2013,
it is now 15th February [ 5 days later] .Does overnight not mean OVERNIGHT? i can understand if it then perhaps takes one day longer, but really, why on earth 5 days! parcel has been tracked and is apparently lying at Rondebosch Cape town for past 3 days now.
Tracking number of parcel is: TC055589811ZA