Depends largely on how experienced the person using them is. They sites can be poorly chosen, too much or too little pressure when squeezing, inconsistent sites when checking at a later date, etc.
How will your training, nutrition, or goal change if you're "really" 15.25% or 19.75% or 17.5%? That's a trick question. It won't.
And that's why, as I've written before, bodyfat percentages are like IQ. The only reason to get it tested is to go around telling people what your score is. There's no other real world impact and it's only good for bragging rights (or to depress yourself, depending on the result).