Most cameras have the ability to take pictures in various quality modes - high quality, medium quality, low quality.
Obviosuly the better the quality the larger the "image" size and the LESS pictures you get on a memory card.
Have you, or anyone else, recently changed the picture quality on the camera (or indeed any other setting).
Have you tried formatting the card (after remving your pictures of course). Maybe there are some pictures still on the card which have got "lost" (so they dont show up) but they are still there.
Formatting the card should get rid of any "lost" pictures.