✔ 最佳答案
Did you reset its EVs with a reset bag?
A pokemon's stats will only ever be its base stats plus IVs, plus EVs, modified by nature and level.
That means if you used a reset bag and reset the pokemon's EVs to 0, then the next time the pokemon's stats were recalculated (at the end of each battle in X/Y), it loses all of the bonus points and its stats will go down. Your Giratina, by the sounds of it, had about 132 points allocated to ATK and 124 in Sp. ATK, so by resetting those to 0, the stats lost the 33 and 31 bonus points.
But as you EV train it again, it will regain the bonus points in whatever stats the EV points are going into now.