I have no idea why, and I also got it from trade.
You only get the pokedex entry (definition) if you catch it yourself AND you didn't have it completely registered in your pokedex. To get it completely registered you have to have previously / currently had ownership of it.
Well, the only thing I can think of is that you have not yet obtained the National PokéDex, so Giratina's PokéDex entry (or 'definition', as you call it) will not appear because it is not in the Kalos PokéDex.
Source: Experience.