I just tested with both ten thousand and a million rows. The results are different now - only IX_Tmp_bob_pkColB is used, independent of index creation order. If I DROP IX_Tmp_bob_pkColB then IX_Tmp_bob_pkColA is used.

Why would it use IX_Tmp_bob_pkColB if I haven't included any reference/data/information about pkColA in it? ("RTFM on covering indices!"?)