The perpage options are not based on how many photos you elect to show. You only have 10 there before you set a cookie because thats how many you elect to show. The perpage options are based solely on multiples of the number of rows this has never changed.
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Content visible to verified customers only.
You will notice that 10 is only there in your situation if there is no perpage cookie set. Once you set a perpage cookie your options are limited to that unless you start a new browsing session.