Photo selection and targeting in Lightroom is indeed one of it's trickier aspects, in my opinion as well. I still make mistakes even after a half-decade of frequent use.
When exporting, it targets all selected photos in the filmstrip, regardless of whether you're viewing grid or loupe, whereas when deleting, and some other ops, it only targets the most-selected photo (when in loupe view).
Not sure what to say, except when exporting - double check the number of photos being exported - it gives the number at top of export dialog box.
Rob
