This is a classic example from philosophy of language.

For centuries, people thought:

the Morning Star (visible before sunrise) was one heavenly body;

the Evening Star (visible after sunset) was another.

Later astronomy showed that both are actually Venus, the same planet.

The reference is the actual thing in the world that both expressions point to.

Likewise,

middle class

middle strata

usually have the same reference (the same people), even though they emphasize those people differently.