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.
Original S1 (Recycle Heaven)
Series 2 (PLS, sold)
TiVo HD (XL, PLS, "652")
Juliet Basic (4XL, PLS)
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!