"Few" and "a few" are significantly different in grammar and meaning.
"A few" is neutral: it simply refers to a small number of whatever, without suggesting any expectations.
"Few" triggers negative polarity expressions (such as "any" and "yet"), and emphasises the smallness of the number, and suggests that a larger number might have been expected.
A few people have registered.
A few people have an idea what's going on.
Contrast this with:
Few people have registered yet.
Few people have any idea what's going on.
both of which are emphasising that the number of people is small.
A few with negative polarity items is not grammatical:
*A few people have registered yet.
*A few people have any idea what's going on.
There is no real answer to your question: like most "why" or "what is the reason" questions about language, the only answer is "because that's the way it is".
1The quantifier few does not always take an article. Few will dispute this assertion! The omission or use of the indefinite article changes the meaning of the usage. In your example, if you omitted the article and wrote I've got few fruits and vegetables, the sense would be that you lacked sufficient produce. With the article, the sense is that you do have sufficient produce. – P. E. Dant Reinstate Monica – 2017-06-09T21:17:35.670