The reason the lisp is often written with a th-sound is that it's pretty impossible to write a fronted [s] with a normal keyboard.

口齿不清经常被误写为th音的原因是因为用一个普通键盘打出前移s音太困难了。

youdao

There's another reason I think these two sound differently and that is the following.

这也是两者听起来的另外一个不同,这是接下来的

耶鲁公开课 - 聆听音乐课程节选