Two people timed an object with a stop watch . they each got a slightly different result how could this be ?

This experiment takes two people. ... 2. The second person starts (and stops) the stopwatch without looking at the display. ... something slightly different for each ... Do the times add up or do they start again from zero each time your finger breaks the .
.. Try it and explain the results. Table 1: Mass in Grams. Object. Mass (g).

Answer///air resistance,drag;