different XP points for stork feathers

I have noticed while visiting neighbors islands that sometimes when I collect a stork feather I get 7XP and other times (during the same play session) I get 5 XP. Why is there a difference?

BTW, this is the FB version I'm playing.

When you get 7 XP that persona has researched the increase XP points science skill - a small benefit for you :-)

OH! Okay, cool! Thanks!