I've also noticed strange behaviour regarding EXIF shooting/modified times in my pics, but I believe it's a Windows problem (or feature?).
I may be completely wrong, but it appears that, let's say, I take a pic in December at 12:00:00 and store in my computer, the EXIF and Windows modified dates will be the same; however, once DST starts in March, it seems Windows changes the dates of all files, so the pic I took in December will still retain its EXIF shooting time, but the modified date will now be 13:00:00. Once the hour changes again, it will revert to the original 12:00:00.