Why does reflection coefficient vary with frequency on transmission lines?

[clubgitmo]

Hey guys - I've been using this website to study transmission lines. (great study aid - posting for others to use. Great for smith charts) However, I've noticed this graph they have for load matching:

I'm having trouble finding or understanding a relationship for reflection vs frequency. I've never used frequency for a reflection coefficient, but clearly there is a relationship. Even for an all real load. Can someone share some wisdom?

 

[zoki85]

Reflection coefficient is some function of frequency too. Apparently it's dependence vs frequency is weak for long, low frequency signals and can be taken for constant. But 1 Ghz...this frequency is pretty high for transmission lines.