", get the code completion, press CTRL+SPACE, and the rest of the code appears. Right? Wrong!
TXC starts at the cursor position, kills "cen" and writes the whole snippet AGAIN thus producing something like
TXC isn't interested in the whole term (e.g. \begin{cen) but only the "cen" part. Thus it only replaces that part and writes the whole term \begin...\end...
That means you only have to write "cen" to get the code hint.