Why did iPhoto remove the jpg suffix from some of my photos?

I had several albums in iPhoto of a recent trip to Scotland. All loaded fine to my iPad. Then I imported some photos from another trip to iPhoto. All looked fine. When I went to import them to iPad got message that iPad didn't recognize format of some of the photos. After making sure I had updates, cleared iPhoto cache, etc. did a fresh sync -- and now some of the Scotland photos wouldn't load, but all from the second trip loaded. Searching various forums I saw that the error message I got resulted from not having jpg suffix at end of each file. So, went back into iPhoto and looked, and sure enough, the files that didn't / wouldn't load had no jpg suffix. Yet they did before! How could that have happened? Fortunately I had copies of the original files on my hard drive and not in iPhoto -- and so I won't have to edit 700 images and add jpg to their label. But I don't want this to happen again ... any thoughts? Thanks.