Mismatch of size of /var/run/utmp and `struct utmp` in Linux built with GLIBC 2.38 (Y2k38 fix)

I am working on an embedded Linux system (kernel-5.10.24), which is a **32bit** system, and it is using GLibc-2.38 to fix Y2k38. The rootfs is built from buildroot rel.2023-aug with Y2k38 fix. (built with -D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE). There are getty and login built from busybox-1.36.1 in the buildroot. Now I found strange things when I tried to read the /var/run/utmp with following codes (also built with -D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE).

#include 
#include 
#include 
#include 

int main() {
    FILE *fp;
    struct utmp ut;

    printf("XXXXXXXXX sizeof utmp: %ld\n", sizeof(ut));
    fp = fopen("/var/run/utmp", "r");
    if (fp == NULL) {
        perror("Error opening UTMP file");
        return 1;
    }

    while (fread(&ut, sizeof(struct utmp), 1, fp) == 1) {
        if (ut.ut_type == USER_PROCESS) {
            printf("Login Name: %s\n", ut.ut_user);
            printf("Login Time: %s", ctime(&ut.ut_tv.tv_sec));
        }
    }

    fclose(fp);

    return 0;
}

When root is logged in and run the code, it showed,

# /tmp/utmpread
XXXXXXXXX sizeof utmp: 400

But the size of /var/run/utmp is 384.

# stat /var/run/utmp
  File: /var/run/utmp
  Size: 384             Blocks: 8          IO Block: 4096   regular file
Device: fh/15d  Inode: 8           Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 1970-01-01 01:14:24.270002128 +0000
Modify: 1970-01-01 00:03:12.000000000 +0000
Change: 1970-01-01 01:12:47.210002082 +0000

So based on my tests, it seemed that struct utmp is already built as 64bit of timeval_t, and the login and getty also report its size is 400Bytes. But the size of /var/run/utmp is still 384 Bytes, which is using 32bit timeval_t. I don't know why there is the mismatch (the code reports size of struct utmp is 400, but the file generated from struct utmp is 384), is it from the GLIBC or from the busybox? Thanks,