If that is a low percentage it is probably because airlines have such massive big budget items on this like airframe depreciation and fuel that perhaps it pushes the IT percentage down compared to say a bank which spends relatively more in percentage terms on IT because it doesn't have to buy things like A 380s or spend a fortune on fuel.
Banks spend a fortune on salaries and bonuses — around 40% of revenue. Yet they still spend 10% on IT.
The difference is probably that banks face a competitive threat from tech startups. In a cashless economy, deposit-taking banks are potentially redundant (and who would mourn them?), but until one of us invents a teleportation device we'll probably still need physical aircraft.