because in your model, the table containing the "backlog_accessories_quantity" field is associated to another table.
This association might cause a duplication in your lines.
Let me try to explain it simpler with an example:
Table 1:
ID
SALES
1
2
Table 2:
ID
DATE
1
DATE1
1
DATE2
When you have a model like this, if you create sum(Sales) for ID=1 it will be 4 instead of 2 because the ID=1 exists in the table 2 in 2 different lines..
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