252 SpA Choice Specs Calyrex-Shadow Astral Barrage vs. 0 HP / 0 SpD Yveltal: 178-210 (45.2 - 53.4%) -- guaranteed 2HKO after Stealth Rock
252 SpA Life Orb Calyrex-Shadow Max Starfall vs. 0 HP / 0 SpD Yveltal: 346-408 (88 - 103.8%) -- guaranteed OHKO after Stealth Rock
0 SpA Lunala Ice Beam vs. 0 HP / 0 SpD Yveltal: 174-206 (44.2 - 52.4%) -- guaranteed 2HKO after Stealth Rock
Those calcs you told mean nothing. Sucker Punch isn't the most reliable, it's just a 50/50 over them going to something else or just staying and blowing you with Astral Barrage. The SpD isn't not only for Calyrex-Shadow, it also sometime soft checks other offensive Yveltal, and the more rarer Lunala which 2HKOes with Ice Beam.
Secondly, Defensive Yveltal also acts as a good Defogger. Its able to Defog on the most common Stealth Rocker in Necro DM, and also Groudon to some extent. If Yveltal isn't Heavy Duty Boots, Rocks + Astral generally gets it chipped and eventually its KOed. As well, no point in Offensive Yveltal to run HDB, as it generally wants Life Orb for the extra damage, and it hits way too feeble without it.
So that's it, it wants to counter Calyrex-S to its full extent which is why its specially defensive.
Hope this helps!