kana - Why is English "ka" and "ca" sometimes rendered as キャ (kya) instead of カ (ka)?

I sometimes see that English ka and ca are rendered キャ (kya) instead of カ (ka). Why is this?

Do English ka and ca really sound more like kya than ka, at least to Japanese listeners? (I know it doesn't to me.)

Answer

In reference to Sawa's request for an example, キャンディ is a case of キャ being used to transcribe English ca.

I asked my Japanese teacher exactly this question many years ago. The reply was that the vowel in English candy is higher (in phonetic terms) than the low front vowel in RP English cast. The fact that キャ is palatalised raises the vowel and makes it sound more like it does in English.

EDIT: Other examples of this effect, for reference, include キャラクター, キャンペーン, キャベツ and others.

It's also interesting that this only happens to velars (キャ, ギャ), so we don't have, for instance, ミャン for 'man'.