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Ivan Derzhanski is almost certainly right.  Ancient peoples would think of
a move of three squares as including the starting square but more modern
people with a better understanding of zero would think of a move of two
squares not including the starting square.  There may be areas of
confusion in ancient sources espaecially compilations from multiple
sources--this might be the real origin of the rule in Tamerlane that the
Bishop cannot move one square, for example.

Comparable examples in other fields: 

Julius Ceasar often sent coded messages using the alphahbetic substituion
A=D, B=E, C=F, etc. He and his contempories described this as advancing
four letters, we would say three.

According to the New Testament, Jesus died on Friday, was in the tomb
Saturday, and rose from the dead Sunday--expressed in the creeds as 'On
the third day he rose from the dead.'  No doubt we would say 'On the
second day . . .' if we hadn't heard it so many times the other way.