quantum field theory - Z and $gamma$ bosons as mixtures of W and B: Part I

When it is said that the photon is "a mixture of W and B" ($B$ being a gauge field associated with the $U(1)$ hypercharge)

I have a question on this:

• When speaking of "mixtures", this is meant as analogous to the quantum mechanics terminology as the linear combination of two density matrices multiplied by classical probabilities? or this is meant in some more esoteric sense?