Grammar justification for 「〜わりに（は）」


I recently came across this expression「〜わりに（は）」 An example:

> りんごは、安い **わりには** 美味しい。([source](http://www.imabi.net/considering.htm))  
>  Apples are delicious, **considering how** cheap they are.

What I don't understand is **why**?

The dictionary entry for わり brings up the following definitions (thanks
jisho.org):

>   1. rate; ratio; proportion; percentage​
>   2. profit​
>   3. assignment​
>   4. 10%; unit of ten percent​
>   5. match; schedule of matches​
>   6. diluted with (of drinks); mixed with
>   7. Wari (place)
>

In the example above, わり modifies 安い, and I just can't understand how to
translate it literally?

One interesting thing is that I found this definition in a J-J dictionary:

> ５ （「…わりに」の形で）ある事から予想・推測される程度。…に応じた程度よりは。「値段の割りに品物がよい」「若い割りには礼儀正しい」

Which loosely translates to:

> (In the form of わりに): More than expected/guessed.

So I'm wondering whether this is just an additional definition to わり (like an
exception the applies in this specific form), or there is some historical
reason, or perhaps a literal translation that makes sense.




I think the わり means the # **1: rate, proportion**. In my opinion, "
**rather** " can work as well.

> リンゴは安い割りには、美味しい：

_Apples are delicious, considering that it **is rated as** "inexpensive" of
the three categories: "inexpensive," "reasonable," and "expensive."_

_Apples are delicious, considering that they are **rather** cheap._

