PL/SQL :: Code Erases Previous Month Record While Updating Current Month Record

Using 11gR2, windows 7 client machine. I need to update the table missing_volume (below), where I need to calculate the estimated_missing column. The calculation of estimated_missing column for current month needs previous month numbers (as commented inside the code below). I want the output like the first table. Notice the records start from January, hence estimated_missing for January can't be calculated, but for the the rest of the months it can be done by simply changing 'yr' and 'mnth' (commented inside the code towards the end).

yr          mnth          location     volume          actual_missing          expected_missing     estimated_missing
---------------------------------------------------------------------------------------------------------------------------------
2013            January          loc1          48037          24               57                         
2013             February     loc1          47960          3660               53                      24
2013             March          loc1          55007          78               57                      28
2013             April          loc1          54345          72               58                  77The code:

UPDATE missing_volume g

[Code]....

The code does calculate correct number for 'estimated_missing' as I run the code for each month, but the problem is while updating the current month it also erases the record for previous month. E.g. as can be seen below, after I updated April the column only has the record for April, previous month record is gone, similarly updating March removed February, etc. I can't understand why it's happening!! Here is the output I get:

yr          mnth          location     volume          actual_missing          expected_missing     estimated_missing
---------------------------------------------------------------------------------------------------------------------------------
2013            January          loc1          48037          24               57                         
2013             February     loc1          47960          3660               53
2013             March          loc1          55007          78               57
2013             April          loc1          54345          72               58                   77

why it's happening (I mean where is the flaw in the code) and how to get the desired output (first table).