Nominal / Z-spread all, why, understand, difference, measures

Iginla2010

I was looking at the Schweser notes pg 115, where they talk about the 2 primary factors that influence the difference between the nominal spread and the zero-volatility spread.
I don’t really understand why the difference between the 2 spread measures will be greater if the spot yield curve is upward sloping. Isn’t the Z-spread a fixed number added to the spot rates, which then tells us the YTM? If so, shouldn’t the Z-spread = nominal spread at all times?
I’m not sure what I missed…any help would be appreciated.