Seems like a simple misspelling that eventually evolved into the official spelling. We've seen this before with system names (Heiligendreuz and New Hati, for example).
In his first introduction to the BT universe (the Liao housebook), he was consistently spelled Tormana. The same spelling persists in Shrapnel (the only source really featuring him in-depth as a protagonist), the 20 Year Update and the 4th Succession War Military Atlas (both volumes).
He's spelled Tormano in Brush Wars, in the CCG.
In Assumption of Risk (the EARC copy supplied by BattleCorps), he's spelled Tormano most of the time but Tormana creeps up at least two times.
Didn't check his appearance in the Warrior trilogy and FM:U because I don't have these as searchable PDFs. In summary, though, it seems the change in spelling crept in when Catalyst took over from FASA.