The following three sentences appear in the same published paper. Why does No. 1 employ the auxiliary "did" whereas the other two omit it?
This could explain why ProRoot WMTA showed significantly darker discoloration than did Angelus WMTA.
Groups 1 and 2 showed significantly greater discoloration than Groups 3–5.
However, PC+BO contained 20% bismuth oxide and showed significantly less discoloration than the two WMTA formulations.