There's nothing wrong with it. It can learn all those moves, and the trainer just gave it a Float Stone. Unusual, but not wrong. Why? We can't answer that.
As for if you should change the moves, well, probably.
While Shadow Force is a fun move and it's Giratina's signature move, it is not recommended you run this if you want maximum efficiency. An opponent could easily switch into something that tanks it while you're charging the move.
Are we looking to be a physical or special Giratina? Thunderbolt is a potential replacement for Charge Beam (unless you like the Special Attack bonus), but there are others. Smogon recommends a stall/Defogger, but that's no fun. I asked around what other options were available, and they said a Calm Mind Giratina will show up every once in a while, and I thought why not give that a shot?
Giratina @ Leftovers
EVs: 252 HP / 252 Def / 4 SpA
IVs: 0 Atk
- Calm Mind
- Aura Sphere
- Shadow Ball