Git has two ways to count modified lines

[Laurent Lyaudet <laurent.lyaudet@gmail.com>]

Hello,

I check the number of lines modified by my commits by hand,
and today I had a surprising count :

1) I commit and get the following count
laurent@laurent-GL73-8SD:~/ReposGit/flow$ git commit
[master c068911] Task CU-21ph8h7 add buttons in PresenceList: "Tout
déplier", "Tout replier", for this the useState for showDetail
(boolean) in Presence is replaced by a useState for showDetails (Set
of ids) in PresenceList; correction missing tr between thead and ths
in PresenceList.
 2 files changed, 88 insertions(+), 48 deletions(-)
 rewrite src/apps/logs/components/PresenceList.js (61%)

2) I check the diff by hand, it doesn't match.
I check on github, it agrees with my manual count.

3) I try to get the same answer again in command-line.
laurent@laurent-GL73-8SD:~/ReposGit/flow$ git log -1 --shortstat
commit c068911547bddbf7bfc4ddc7a68ee8482421ed5c (HEAD -> master,
origin/master, origin/HEAD)
Author: Laurent Lyaudet <laurent.lyaudet@gmail.com>
Date:   Wed Mar 16 18:40:25 2022 +0100    Task CU-21ph8h7 add buttons
in PresenceList: "Tout déplier", "Tout replier",
    for this the useState for showDetail (boolean) in Presence is replaced
    by a useState for showDetails (Set of ids) in PresenceList;
    correction missing tr between thead and ths in PresenceList. 2
files changed, 71 insertions(+), 31 deletions(-)

How comes git has two ways to count modified lines ?
How can I make git output again the same numbers than just after commit ?
How can I check how it computed these numbers in the first place ?

Thanks, best regards,
    Laurent Lyaudet