Peter, I can only say that I have had a Kudu that has lost no lenght since the day I shot him until now, 8 years later. I have a few other species that are the same. Did you measure the horns before they went off to the taxidermist? Another thing to consider and it still happens to me is that in that perticular moment the trophy animal is shot, everyone is very excited and pumped up, the animal always seems bigger the longer you look at it. The other thing is that photos are taken by the PH to show the trophy to iys best advantage, sometimes a little trick photogarphy makes the trophy look much larger.
It is also not so uncommon for taxidermist to mix up horns,its happend to me once, I changed my taxidermist. My best advice is always take specific photos of the horns,cape or anything else that can ID your trophy. Another species that causes some ID issues is Zebra, the skin you hand in and the one you get back often seem to be 2 different animals. I have also come across a friends trophy were some of the horn at the base was removed by the taxidermist, I cannot tell you why, maybe to make it fit better.