Why Is It?

Why is it that the two words "sign" and "significance" are pronounced so differently?
We say "sEYEn" but not "sEYEnificance"...

They're related you know.

The OED confirmed my suspicions that "sign" is the one that is pronounced differently than its roots and origin would seem to dictate... By all rights it ought to be "sIG-en". I rest my case.