and the number of items in your gallery exceeds 10. According to Shadowbox's options :
"counterLimit // Limits the number of counter links that will be displayed in a "skip" style counter. If the actual number of gallery elements is greater than this value, the counter will be restrained to the elements immediately preceding and following the current element. Defaults to 10."
This means the counter acts as a dial, the currently viewed image is always in the center: