FAT (Fully Automatic Timing) vs. Hand Timing:

FAT (fully automatic timing) uses a computer and a camera to time races. FAT times are considered accurate and require no conversion. Hand times, those with humans operating the stopping and/or starting mechanisms, are highly prone to error and generally produce faster times. By rule, they are only accurate to a tenth (.1) of a second. All 100ths of a second beyond zero must be rounded to the next highest tenth (IAAF Rulebook Rule 165 10a). Many track and field statisticians use a conversion factor estimate of 0.24 seconds added to any hand-timed mark in the 100 m or 200 m event, and 0.14 seconds to any hand-timed mark in the 400 m or longer event. This allows hand timed marks to be compared to FAT times (Wikipedia)