Kelly Scott and Corrected Vision:What's the Decision Rule?

I noticed at The Scott that sometimes Kelly Scott, skip of Team Canada, wears contact lenses; other times she wears glasses.

In the opening draws of the The Worlds, she was wearing contacts; but since then I think she has been wearing glasses.

However, I haven't paid really close attention to when she wears which. Does anyone know why or how she decides to wear which when?