It's common in English to use these types of statements where a direct answer would seem too assertive. For example, if you'd asked me how I had been lately, I'd likely say "not bad", rather than "good", as being good implies being better than normal, whereas not bad is just not bad.
People often say that things are not the opposite of what they are. They do this to avoid seeming forward and assertive when stating that something is the case.
Other examples include
- Not the best (bad)
- Not the brightest (stupid)
- Not exactly perfect (flawed)
- Not without its problems (problematic)
- Not exactly rocket science (trivial)
English speakers often avoid making assertive statements unless they fully intend to be assertive. For example, they will often begin a statement of fact with "I believe...", "It seems...", "Apparently...".
I guess this gives assertions more impact too. To say "He is stupid" is far more offensive than saying "He is not the brightest chap I've ever met."