In Aperture, the keyword count can include images that a smart album view may not currently show.

Based on the reported causes, check these first:

• photos in the Trash
• rejected images, which may be hidden by current filters
• images inside stacks, where only one image appears if stacks are collapsed
• images in shared sources/services such as Photo Stream, Facebook, or Flickr

So the “missing” photos usually aren’t gone—they’re just excluded from the current view or located in places the smart album/browser presentation doesn’t reveal directly.

Try expanding stacks, clearing any reject/status filters, checking the Trash, and looking in shared-service sections of the library. In at least one case, the discrepancy was caused by keyworded images stored in shared services rather than the main Photos view.