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-- run in Windows 10
1 - 
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')
f.seek(0)
f.readline()
print(f.tell()) # output 5 (xxx\r\n)
x = f.truncate()
print(x) # output 13 (xxx\r\nyyy\r\nzzz), why it is 13, not 5?
2 - 
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')
f.seek(0)
f.readline()
print(f.tell()) # output 5 (xxx\r\n)
f.seek(f.tell())
x = f.truncate()
print(x) # output 5

# Run in Windows 10
# Code snippet 1
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')
f.seek(0)
f.readline()
print(f.tell()) # 5
x = f.truncate()
print(x) # 13 - why it is 13, not 5?
# Code snippet 2
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')
f.seek(0)
f.readline()
print(f.tell()) # 5
f.seek(f.tell())
x = f.truncate()
print(x) # 5

This is the same as Issue 26158. Truncating text files is not clearly documented for a start, and truncating after reading doesn’t seem to be considered much in the implementations.
Your question is answered at <https://bugs.python.org/issue26158#msg258619>. Your code calls the C implementation of "io.TextIOWrapper.truncate". This implementation does not consider that there are 8 unread bytes in a buffer, so it truncates the file at the end of the buffer (5 read + 8 unread = 13 bytes).