Plotting a function with units gives two different results

With the following code block, I plot a function with units in it in two different ways. Both ways should do the same in my understanding of the documentation.

cm = Quantity["Centimeters"];f[x_] := Sqrt[x + 5 cm]Plot[Evaluate@{f[zz cm]}, {zz, -5, 5}]Plot[Evaluate@{f[zz]}, {zz, -5 cm, 5 cm}]Plot[Evaluate@{Sqrt[zz]}, {zz, -5 cm, 5 cm}]

Can anyone explain why it does that?