with语句在多线程编程中遇到的问题

代码段1：

# -*-coding:utf-8 -*-
from threading import Thread, BoundedSemaphore
from datetime import datetime
import time

# 定义一个函数
def showtime(signal):
    print(datetime.now())
    time.sleep(2)
    signal.release()

# 创建一个有界信号量对象，内部计数器的初始值为3
bsema = BoundedSemaphore(3)
# 创建并启动10个线程
with bsema:
    for n in range(10):
        bsema.acquire()
        t = Thread(target=showtime,args=(bsema,))
        t.start() 

代码段2：

# -*-coding:utf-8 -*-
from threading import Thread, BoundedSemaphore
from datetime import datetime
import time

# 定义一个函数
def showtime(signal):
    print(datetime.now())
    time.sleep(2)
    signal.release()

# 创建一个有界信号量对象，内部计数器的初始值为3
bsema = BoundedSemaphore(3)
# 创建并启动10个线程由于信号量初始值为3,所以每次只有3个线程并发执行
for n in range(10):
    bsema.acquire()
    t = Thread(target=showtime,args=(bsema,))
    t.start() 

代码段1的结果是每次只有2个线程在执行函数showtime，但是代码段2的结果是每次有3个线程在执行showtime函数，原因是什么？

代码段1执行结果：

2022-02-06 00:35:46.583684
2022-02-06 00:35:46.583684
2022-02-06 00:35:48.598238
2022-02-06 00:35:48.599237
2022-02-06 00:35:50.607590
2022-02-06 00:35:50.608596
2022-02-06 00:35:52.622436
2022-02-06 00:35:52.623437
2022-02-06 00:35:54.630541
2022-02-06 00:35:54.631551

代码段2执行结果：

2022-02-06 00:32:59.856888
2022-02-06 00:32:59.857888
2022-02-06 00:32:59.857888
2022-02-06 00:33:01.862122
2022-02-06 00:33:01.863158
2022-02-06 00:33:01.864120
2022-02-06 00:33:03.865883
2022-02-06 00:33:03.866844
2022-02-06 00:33:03.867841
2022-02-06 00:33:05.878067