Why all output of my C program directed to stdout is sent to journald?

I work on yocto based linux distribution. I'm not expert about systemd and neither about systemd-journald. My C program my_c_program is started at boot by the script my_script.sh. Below there is the file my_script.sh:

my_c_program &
echo $! > /dev/shm/my_c_program.pid

Previous script is started by a service. The unit file of the service is called start_c_program.service. Below there is the unit service:

[Unit]
Description=Start my_c_program
Requires=...
After=...
Before=...

[Service]
Type=forking
ExecStart=/usr/bin/my_script.sh
PIDFile=/dev/shm/my_c_program.pid

[Install]
WantedBy=multi-user.target

With this configuration all instructions printf("message") present in my_c_program add the string "message" to systemd-journald. I know that if I modify my_script.sh to:

my_c_program > /dev/null &
echo $! > /dev/shm/my_c_program.pid

the message on journald are suppressed, but the question is: why all output of my C program directed to stdout is sent to journald? Thanks ___ I was thinking that to send message to journald I had to use the C instruction syslog(), for example by a code as below:

#include
int main(int argc, char** argv) {
    syslog(LOG_INFO, "Start logging");
}