Why would reticulocyte counts be increased in chronic blood loss, but decreased in Vitamin B12 deficiency?

Retics are immature RBCs.  If blood is chronically being lost, the bone marrow is constantly putting out RBC's.   The marrow cannot make the mature ones fast enough and often sends out the retics to make up for the low count.  

I don't remember why it is low with B12 issues.

I saw your other blood question too.  What are you studying for?

Reticulocytes increase in blood loss as a compensatory measure.  Reticulocytes are immature RBCs, so their presence reflects an attempt by the bone marrow to produce more RBCs (driven by EPO -- erythropoietin).  

B12 is required for DNA synthesis and cell division, so synthesis of RBCs would be impaired.  In fact, with folate or B12 deficiencies, you may observe a macrocytic anemia -- few red blood cells of larger than normal size.  In this situation, the RBCs can grow and produce hemoglobin, but they can't divide, so they just swell in size.