Again, the answer lies in backward compatibility.

答案同样是因为向后兼容性。

youdao

An owl's head, for example, can twist so far round that it faces backward, but when the bird wants to look forward again, it must turn its head back the way it came.

比如说猫头鹰可以将正脸一直转到后脑勺的位置，但是如果它再想看到正前方的东西，那整个头部只能原路返回了。

youdao

The way he was moving was not like a normal person would, it was like a glitching computer; he would "jump" forward, backward or to a side, then back to his normal place again.

他走的样子不像是个正常人，像个短路的电脑似的，他“跳著”向前、退后、或是向侧面走，然后又退回到之前站的位置。

youdao

Then it runs forward three weeks again and then backward, continuing to improve its estimate.

FORBES: Magazine Article

They are bending over backward again to please them.

FORBES: Ernst & Young Deaf, Dumb, And Blind About News Corp.

Through the bare windows the sun shone, lighting up the floors as if her life were going backward and she was again a young bride in a new-built unfurnished manse and with a somewhat frightening husband twice her age.

NPR: E.L. Doctorow on Sherman and 'The March'