为什么 process_time 总是小于 perf_counter？

import time
a1=time.perf_counter()
a2=time.process_time()
a3 = time.clock()
print(a1)
print(a2)
print(a3)
c=1
for i in range(1,200000):
    c*=i
b2=time.process_time()
b1=time.perf_counter()

b3=time.clock()
print('b1-a1=',b1-a1,'s')
print('b2-a2=',b2-a2,'s')
print(b3-a3,'s')

1.642092074565759e-06
0.9984063999999999
7.799937354187355e-06
b1-a1= 36.65670751507543 s
b2-a2= 34.3514202 s
36.656702588799206 s

b2-a2总是小 2秒左右，怎么回事？