Apparently, if this person is right, rounded loaves were the norm until around 60 years ago, and the oblong tins we use were once called sandwich tins.
Apparently one meaning of 'round' is a circular slice.
So, the meaning of 'round' for sharing has stuck with sandwiches, whilst the meaning of 'round' as a circular slice has stuck with toast.
http://uk.answers.yahoo.com/question...9061310AAhw4Ld