When you don't put EVs into these calculators, they assume all of your Pokemon's EVs are 0. 158 to 178 is indeed the legal range for Gardevoir's HP at level 63 with 0 EVs. If your Gardevoir does have EVs, then the calculator is going to give strange-looking results.
For your reference, I've listed the legal range for Calm Gardevoir's stats at level 63. The lower bound uses 0 IVs with 0 EVs and the upper bound uses 31 IVs with 255 EVs. (The upper bound stats cannot occur simultaneously because there cannot be 255 EVs in every stat.)
- HP: 158 to 217
- Atk: 77 to 131
- Def: 86 to 146
- SpA: 162 to 221
- SpD: 163 to 229
- Spe: 105 to 165
Using IV calculators is a waste of time if you don't know your Pokemon's EVs. To get a helpful result, there must be only one variable: IVs.