pthread Read-Write-lock 질문

#include
#include
#include
#include
#include
#include

pthread_rwlock_t R_Lock;
int ac=0;

void* RD_LOCK(void* arg) {

pthread_rwlock_rdlock(&R_Lock);
printf("%d\n",ac); // ac is global variable 0
sleep(1);
pthread_rwlock_unlock(&R_Lock);
}

void* WR_LOCK(void* arg) {

pthread_rwlock_wrlock(&R_Lock);
printf("RW LCOK\n");
ac++;
printf("%d\n",ac);
sleep(1);
pthread_rwlock_unlock(&R_Lock);
}

void DO_RW_LCOK(){
pthread_t RW[3];
pthread_rwlock_init(&R_Lock, NULL);

pthread_create(&RW[0], NULL, RD_LOCK,NULL); //==0 thread
pthread_create(&RW[1], NULL, RD_LOCK,NULL); //==1 thread
pthread_create(&RW[2], NULL, WR_LOCK,NULL); //==2 thread

pthread_rwlock_destroy(&R_Lock);

for(int i=0; i<3;i++)
pthread_join(RW[i],NULL);
}

int main(){
DO_RW_LCOK();
return 0;
}

순서를 read(0번 1번)를 다하고 write lock을 걸면 write가 우선실행 되던데 이유가 뭔가요?