I think they've got a slightly different definition of retaining to me....
"Republic Act No. 9225 or the Citizenship Retention and Re-acquisition Act of 2003 is a law passed on 29 August 2003 which grants natural-born Filipinos who have lost their Filipino citizenship through naturalization in a foreign country, the opportunity to retain or re-acquire their Filipino citizenship."
What they seem to mean is anyone who has only just lost their Filipino citizenship is 'retaining' it, and anyone who lost their Filipino citizenship a while back (ie before this law was granted) is 're-acquiring' it. In either case though the Filipino citizenship would be lost for a period of time.