I've noticed that, in Diamond, when I use Light Screen or Reflect, the message will vary. Sometimes it says, "Light Screen/ Reflect raised your team's Sp. Def/ Def slightly!", but sometimes it doesn't say 'slightly'. How come?
It will say slightly if you are in a double battle, in which your team's (special) defence is 25% more instead of 50% more. I hope this helps! -Jellohamster
