From how I understand it [as a non native of Russian],

All question words in languages can be broken down into "pure" forms using more basic question words, usually bottoming out around the king of all question words "what". как can be broken down into "In what way?" whereas насколько into "to what extend", "at what amount" (or even "on what level", noting the на in насколько). Seeing as сколько is "what amount of" this would make some sense.

Насколько being three syllables as opposed to the one of как, as well as the somewhat abstract difference between them, I'd imagine some linguistic "slouching" has arisen regarding the use of насколько, making как used instead at times.

In english, since we haven't really a word to distinguish the two meanings, "how" is used for both(if at very least for ease).