Hi,

 

If two individuals measured the same 3 objects and recorded the weight each time, and have slight differences in every one of thier weights, would this be a result of rounding or not setting the scale to 0? (I think it's the latter but need confirmation).

Both, are plausible, plus a whole lot more.

• The scales might have a scale factor error. A scale doesn't measure weight directly. It instead measures the vertical deflection of a spring. The analog (or in some cases now, digital) translation from deflection to weight assumes a specific value for the spring constant. The true value might vary from the assumed value. The true weight is the measured weight times some factor. This non-unity factor is called a scale factor error and is present in many kinds of measuring devices.
  
• One way to overcome scale factor error is to calibrate the device around a known value. This can be done with a spring scale by adjusting the zero point to read say, 100 pounds when a known 100 pound weight is placed on the scale. While this will make the scale give a more accurate reading for objects that weigh 100 pounds, it will increase the error for objects that weigh considerably less than this. In particular, the reading will be non-zero when nothing is on the scale.
  
• Thus use of different calibration procedures or different standard masses will lead to different readings.
  
• Operator error is another source. A person who looks at the scale from slightly to the left of center will get a different reading from someone who looks at the scale from slightly to the right of center, and both will get a different reading from someone who properly looks straight down at the dial. There are lots of other places where operator error can creep in.
  
• Just to name a few other sources of the difference: Did they weigh the objects in different places on the Earth? Did one use a spring scale and the other use a balance scale?