UTF8Encoding string to byte []转换意外行为

I have this piece of code:

我有这段代码:

byte[] bytes = ...

// Here my bytes.Lenght is 181 (for example)

var str = UTF8Encoding.UTF8.GetString(bytes);
bytes = UTF8Encoding.UTF8.GetBytes(str);

// Here my bytes.Lenght is 189
byte[] b