It doesn't. The 3rd element is a string 3 4
-- the braces you see are not actually part of the string. You're joining with a space, so the space in the string is not visually different. An example:
% lappend aaa 1
1
% lappend aaa 2
1 2
% lappend aaa {3 4}
1 2 {3 4}
% lappend aaa 5
1 2 {3 4} 5
% join $aaa :
1:2:3 4:5