There may be no "explanation" (sometimes learning a language means just accepting things as they are and memorizing away), but is there any reason why "nog" means "still," but "nog steeds" also means "still"?
Similarly, is there any reason why "normaal" means "usually," but "normaal gesproken" also means "usually"?
The problem, is that you're trying to use a single English word to describe a Dutch word with slightly different meaning in those exercises.
Nog and Steeds don't directly translate their full meaning into English, but with context come close.
I advise you take a look at the following references:
"nog steeds" entails duration or repetition. Er liggen nog vier boeken op de tafel - There are four books left on the table ("nog" brings an expectation of the amount to shrink) Er liggen nog steeds vier boeken op de tafel - Four books are still laying on the table. (/Someone/ has to come pick them up)
Heb jij nog een pen voor mij? - Have you got a pen for me (by chance)? ("nog", unless stressed in which case it means "another", merely indicates the likeliness of the asker having tried several others before) Heb jij nog steeds een pen voor mij? - Is it still the case you can borrow me a pen? Only usable if you had asked before and didn't take it then.
There is no such sharp distinction between 'normaal' and 'normaal gesproken'. The latter takes more time so sounds more impressive, generally :) A couple of participles can be fitted to normaal, if used adverbially of course: 'normaal gezien', 'normaal gezegd', 'normaal bekeken', 'normaal beschouwd'... 'normaal gesproken' is by far the most frequent of these.
Just one example comes to mind: "Daar had ik normaal gesproken wel aan gedacht". I wouldn't leave out 'gesproken' there.