It sounds like the Tivo is deleting the oldest show and recording a new show to replace that, keeping it at 5. If the shows were set as "Keep Until I Delete" then it will stop recording when it reaches 5 shows.
Series 2 (Retired, sold)
TiVo HD 1TB (157 HD Hrs, PLS, "S3HD")
Roamio, Oh Roamio!
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!