Why 'git commit --amend' generates different HEAD sha1 each time when no content changes

AFAIK, commit sha1 is only determined by commit object content (say parent commit, tree sha1 and so on). So why 'git commit --amend' changes the commit sha1 when no content changes as following shows.

$ mkdir A && cd A && echo foo >foo

$ git init && git add foo && git commit -m 'add file foo'

Created initial commit 8626800: add file foo
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 foo

$ git commit --amend
Created commit 3591035: add file foo
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 foo

$ git commit --amend
Created commit 3927d9a: add file foo
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 foo