iPhone recently deleted photos has wrong number

My Recently Deleted Photos album has 135 photos in it yet if I click Select then Delete All, it says 'Delete 492 Photos'.

Why are the two numbers not the same? I recently permanently deleted lots of photos by mistake that I'd like to recover, probably around 300 in fact, does this mean that they may still be on the phone somewhere and recoverable? I'd assumed they were gone forever. I've got iPhone 6 on 8.3

Thanks