Order of Execution of Set clause in Update Query (Postgres)
There is nothing "strange" about this behaviour - it's the only sane way to process such an update. 

There is no such thing as "order of updates" - all SET clauses should be assumed to happen in parallel. 

The SQL standard requires that the right hand side of the assignment is the value of the columns as it was before the UPDATE statement started. The order in which the column assignments are listed in the UPDATE statement is irrelevant for the outcome of the UPDATE statement.

Any other behaviour of the UPDATE statement would be a bug and a violation of the SQL standard.

For the same reason, the following statement will swap the two column values:

update some_table
   set x = y, y = x;


or 

update some_table
   set y = x, x = y;