The name "Octavian"

[Jona Lendering]

Why should Scipio Africanus be the exception?

Because I do not know of many other examples. I did a run through the Fasti Consulares (the only easily accessible dbase I have), and noticed the following -ianus names:

329 C. Plautius Decianus
322 Q. Fabius Maximus Rullianus (or Rullus)
179 L. Manlius Acidinus Fulvianus (adopted, according to Velleius Paterculus, Epitome of Roman History, 2.8 )
147 P. Cornelius Scipio Aemilianus
145 Q. Fabius Maximus Aemilianus
142 Q. Fabius Maximus Servilianus
131 P. Licinius Crassus Dives Mucianus
77 Mam. Aemilius Lepidus Livianus
72 Gn. Cornelius Lentulus Clodianus
61 M. Pupius Piso Frugi Calpurnianus
42 G. Vibius Pansa Caetronianus
23 L. Sestius Quirinalis Albinianus
12 M. Valerius Messalla Appianus
9 T. Quinctius Crispinus Sulpicianus
2 AD T. Quinctius Crispinus Valerianus
7 A. Licinius Nerva Silianus
8 Sex. Nonius Quinctilianus
31 L. Aelius Seianus
32 L. Arruntius Camillus Scribonianus
35 M. Servilius Nonianus
38 M. Aquila Iulianus
38 Sex. Nonius Quintilianus
39 L. Apronius Caesianus
45 Ti. Plautius Silvanus Aelianus
45 M. Pompeius Silvanus Staberius Flavianus

After this, the tria nomina system starts to disappear. Of this list, we have evidence for only two men that they were adopted (bold). I think that's meagre, but there may be some antiquarian notice in, say, Gellius or Varro that -ianus-names indicate adoption.