Thank you for your detailed answer. The NOC code is 5241 Graphic designer, and the targeted are is Gibsons (BC, Sunshine coast)There is no official requirement for funds for the self employed persons stream, but I think it all depends on your NOC and your intended occupation stated in Schedule 6A (or business plan) and your intended place of residence.
Say, the funds required for a writer intended to live in Charlottetown (PEI) would probably be significantly lower than... a film maker intended to live in Toronto?
Your lawyer/ agent may actually have rationalized the budget based on your background, so that your profile are more convincing in terms of 'demonstrating the intention and ability to create his/her own employment in Canada'.
Just my guess.
May I ask what is your NOC, and where is your intended place of residence in Canada?
20K for a family of 3 is the basic funds required for skilled workers without a valid job offer.