span:nth-child(2n+1), WITHOUT an <em> among the child elements.

Children 1, 3, 5, and 7 are selected.

Span 1! Span 2 Span 3! Span 4 Span 5! Span 6 Span 7!

span:nth-child(2n+1), WITH an <em> among the child elements.

Children 1, 5, and 7 are selected.
3 is used in the counting because it is a child, but it isn't selected because it isn't a <span>.

Span! Span This is an `em`. Span Span! Span Span! Span

span:nth-of-type(2n+1), WITH an <em> among the child elements.

Children 1, 4, 6, and 8 are selected.
3 isn't used in the counting or selected because it is an <em>, not a <span>, and nth-of-type only selects children of that type. The <em> is completely skipped over and ignored.

Span! Span This is an `em`. Span! Span Span! Span Span!