I see what happened now. The opposing Gardevoir used
Calm Mind. Twice. Once on 2/9 and again on 4/9.
Idk if you IV breed for or how many perfect IVs you go for if you do, but I can see from the video that your Steelix has only 6 HP IVs, which just happened to make the difference here (and I have no idea if it's Sp.Defense IVs are perfect or not either, but the calc assumes that):
With your Steelix's HP IV: +2 252 SpA Gardevoir Focus Blast vs. 0 HP / 0 SpD Mega Steelix through Light Screen: 139-164 (100.7 - 118.8%) -- guaranteed OHKO
With a Steelix with a perfect HP IV: +2 252 SpA Gardevoir Focus Blast vs. 0 HP / 0 SpD Mega Steelix through Light Screen: 139-164 (92.6 - 109.3%) -- 56.3% chance to OHKO
With the Gardevoir living the Iron Head, it probably is one with high Defense investment, and you may have gotten unlucky on the damage roll:
252+ Atk Mega Steelix Iron Head vs. 4 HP / 252+ Def Gardevoir: 138-164 (95.8 - 113.8%) -- 81.3% chance to OHKO
Though I don't know what your Steelix's nature and Attack IVs are, and if they're less than perfect or the nature's not +Attack that may have effected things as well.