One logical thing I can think of, is that the camera took the pictures on its internal memory.

You can find out by removing the memory card from the camera and then turn-on the camera and look at the photos left in the camera.
With the memory card removed you will be seeing internal memory photo files only.

See if you find your missing photos on internal memory.

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Another possibility:
You are using a memory card that has too much capacity.

Older cameras can not read SDHC (SD High Capacity)
A SDHC card is any card that has a capacity of 4 gig or greater.

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Another possibility:

You are using an older card reader with a SDHC memory card.
Older card readers cannot see all the memory on SDHC memory cards.


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