请问为什么低优先级会影响高优先级线程呢

/ 线程1 的入口函数/
static void thread1_entry(void *parameter)
{
rt_uint32_t count,i,j,k = 0;

while (1)
{
Display();/18 tick/
k ^= 1;
if(k)
GPIO_WriteBit(GPIOC,GPIO_Pin_5,1);
else
GPIO_WriteBit(GPIOC,GPIO_Pin_5,0);
rt_thread_mdelay(100);
}
}

ALIGN(RT_ALIGN_SIZE)
static char thread2_stack[4096];
static struct rt_thread thread2;
/ 线程2 入口/
static void thread2_entry(void *param)
{
rt_uint32_t count,i,j,k = 0;
while (1)
{
count++;
count %= 160;
Line(count,1,0xFF);

i = 15;
while(i--)
{
LCDPaint(i*16 ,(count/16)16, 16, 16, &GB2312[32(15-i)],0);
}
if(key_flag)
{
Task();
key_flag = 0;
}
k ^= 1;
if(k)
GPIO_WriteBit(GPIOC,GPIO_Pin_4,1);
else
GPIO_WriteBit(GPIOC,GPIO_Pin_4,0);
rt_thread_mdelay(50);
}
}

ALIGN(RT_ALIGN_SIZE)
static char thread3_stack[4096];
static struct rt_thread thread3;
/ 线程3 入口/
static void thread3_entry(void *param)
{
rt_uint32_t count,i,j,k = 0;

while (1)
{
KeyScan(); /* 每10ms扫描按键一次 */
k ^= 1;
if(k)
GPIO_WriteBit(GPIOC,GPIO_Pin_6,1);
else
GPIO_WriteBit(GPIOC,GPIO_Pin_6,0);
rt_thread_mdelay(10);
}
}

/ 线程示例/
int thread_sample(void)
{
/ 创建线程1， 名称是thread1， 入口是thread1_entry/
tid1 = rt_thread_create(“thread1”,
thread1_entry, RT_NULL,
THREAD_STACK_SIZE<<4,/THREAD_STACK_SIZE,/
25, THREAD_TIMESLICE);

/* 如果获得线程控制块， 启动这个线程*/
if (tid1 != RT_NULL)
rt_thread_startup(tid1);
/* 初始化线程2， 名称是thread2， 入口是thread2_entry /
rt_thread_init(&thread2,
"thread2",
thread2_entry,
RT_NULL,
&thread2_stack[0],
sizeof(thread2_stack),
25 - 1, THREAD_TIMESLICE);
rt_thread_startup(&thread2);
/ 初始化线程3， 名称是thread3， 入口是thread3_entry */
rt_thread_init(&thread3,
"thread3",
thread3_entry,
RT_NULL,
&thread3_stack[0],
sizeof(thread3_stack),
25 - 2, THREAD_TIMESLICE);
rt_thread_startup(&thread3);
return 0;
}

int main(void)//PRIORITY：10
{
rt_uint32_t count = 0;

thread_sample();
while (1)
{
rt_thread_mdelay(100);
}
}

优先级：main>thread3>thread2>thread1
但是thread3的输出PC方波容易被打乱，周期不是10ms
thread2的输出PC方波周期也不是50ms，
thread1的耗时Display();/18 tick/屏蔽则所有任务都正常，

请问为什么低优先级会影响高优先级线程？对于优先级和释放时间的分配策略有哪些？
谢谢