SQLite - xColumn called twice for a value

Link: SQLite - xColumn called twice for a value

<blockquote class="link_og_blockquote">xColumn called twice for a value. Hi Suppose I have a virtual table v with a column c. Then, if I do: SELECT c, c+1, c-1 FROM v; xColumn() will then only be called once per row. Which is...</blockquote>

Virtual Tableについてすこし調べていたら、どうも値を取ってくるときにxCloumnの関数を２回呼んできて謎だった。
どうやら仕様らしい？