Intuitively, I feel that the block must add a wider scope than the expression

I think you mean narrower (i.e more restrictive) scope. Lexicals declared in the block are not visible outside of the map. Lexicals declared in the expression are visible outside of the map

I agree. That narrower scope is the compile-time scope I mentioned earlier. map BLOCK has one. map EXPR, doesn't (unless EXPR includes a do{} or something).