It is easier to see what is going on with Sum and evaluation if you use
Makelist instead of Sum:
(C1) makelist(q,i,1,3);
(D1) [q, q, q]
(C2) q:i^3;
3
(D2) i
(C3) makelist(q,i,1,3);
3 3 3
(D3) [i , i , i ]
(C4) makelist('(q),i,1,3);
(D4) [q, q, q]
(C5) makelist(ev(q),i,1,3);
(D5) [1, 8, 27]
This is all rather confusing, but perhaps internally consistent. Even
more confusing when you add subst or buildq and ev to the mix. But that
is another story.
-s