Inconsistent Piston behavoir

What I expected to happen was...:
When a piston is retracted or extended, it should always take the same amount of time to retract or extend the piston. For example extending a piston takes always 2 ticks, no matter how often you will try.
 
What actually happened was...:
The time which a piston takes to extend or retract varies.
 
Steps to Reproduce:
1. Build a contraption which measures the time, which is taken to extend or retract a piston.
      1a. Use commandblocks with the command "/time query daytime" 
      1b. Use the same delay on both sides (here I used observers, pic1)
2. By reading the output in the chat, the delay varies from 2 to 4 to 6 ticks.
      2a. Results: 1. run --> 2 ticks; 2. run --> 4 ticks; 3. run --> 2 ticks; 4. run --> 3 ticks; .....