User blog comment:ZPPHR/Zane/@comment-79.24.168.66-20121112090550/@comment-5638912-20121112120855

That's simple. Because you only get to choose two Sigils, one at level 6 and the other at level 18. When you have a level 6 Sigil it's gonna have its weakest effect, but when you choose the second level 18 Sigil, both will scale to their strongest effect. That way you don't get the strongest effect at low levels.

But now that I'm thinking about it, I'm gonna change it to levels 8 and 16. :P