[kirbyzhou]

In KODI GUI, I can see there are 872 movies in my collection.
But when I open the underly database, I can see 971 movies;

mysql> select count(*) from movie \G
*************************** 1. row ***************************
count(*): 981
1 row in set (0.00 sec)

Why they are different?

[Karellen]

Maybe you have Music Videos or Concerts that are displayed in the Music Videos tab, not the Movies tab.

[Klojum]

Why they are different?

How many movie sets are there in your movie collection, assuming that display option is enabled.
That SQL query does not deal with movie sets, so it would display the total number of movies.

[DaVu]

In KODI GUI, I can see there are 872 movies in my collection.
But when I open the underly database, I can see 971 movies;

Jepp...as @Klojum said. Most likely due to grouping movies in sets.