The delay in the command block is not accurate to real world time

As can be seen in the video, when you set a scoreboard, display it, and then set the delay of the last command box to 20, but the 20 game tick is not exactly equal to 1 second. When I compared the exact real time for about 10 minutes, I found that the number of seconds displayed in the game is not exactly equal to the number of seconds in the real time. (Because of the limitation of the video size, I did not put the video completely.)
In another word, when the second value in the scoreboard named tick I set in the game is 600, that is, the 10 minutes calculated by the game (because 600 seconds is equal to 10 minutes), However, if we compare the real time or timer with the values I listed in the game, we find that there is an error, that is, 20 game tick equals 1 second, which is not accurate
[Sorry, this is translated into English by a translator]
So the problem I want to solve is that 20 game ticks are inaccurate and equal to 1 second.
How to realize this step? 1. Enter the command/scoreboard objects add tick dummy (pulse, unconditional, red stone required, delay 0) in the command box. 2. In the command box, enter/scoreboard objects setdisplay sidebar tick (Pulse, unconditional, red stone required, delay 0). 3. Enter/scoreboard players add second tick 1 in the command box (cycle, unconditional, red stone required, delay 20).
How did I find this problem? Cause: I want to display the time in the real world in my world. When I finished all the command blocks, the next day I found that the time did not exactly match the real world. Therefore, I conducted the experiment I mentioned just now to explore whether the 20 game ticket is exactly equal to 1 second in reality.