The difference is the bit rate they use.  The bit rate affects how clear the videos looks, especially in scenes where there is motion.  The camcorder uses a variable bit rate (VBR) when encoding the video data so the actual recording times will vary depending on the content of the scenes.  Recording in LP mode gives you a longer recording time; however, depending on the scene being recorded, the video and sound recorded in LP may be distorted.  We recommend using HXP or XP recording modes for better quality.