Blizzard has increased the Dragon Soul buff/debuff from 20% to 25% during this week's maintenance:
At the completion of scheduled server maintenance during the week of June 5, the “Power of the Aspects” spell will grow more powerful, reducing the health and damage dealt of all enemies in the Dragon Soul raid by 25%. This spell has grown progressively stronger over time to reduce difficulty and make the encounters more accessible. The spell will affect both normal and Heroic difficulties, but it will not affect the Raid Finder difficulty.
If you don’t need the help of the Dragon Aspects, the spell can be disabled by talking to Lord Afrasastrasz at the beginning of Dragon Soul.
We hope you continue to enjoy Dragon Soul, and that these changes encourage you to attempt a higher difficulty, or just keep pushing to take down that next boss. source