What is different about LOOKUP and SUM?
Determining whether to use SUM or LOOKUP depends on how the mapping table is set up between the Source List (where the original data resides) and the Target List (where the formula pulling the data resides).
A mapping table between the Source List and Target List can be set up in one of two places: (1) in a Module, using a List-formatted Line Item or (2) in a List on the Settings tab, using a List-formatted Property.
If your mapping table contains the Target List and maps to a Source List-formatted Line Item or List-formatted Property, then use LOOKUP in your formula. Setting your mapping table up this way says that each Target Item can map to one and only one Source List Item in the mapping table, so we are looking up that Source Item value. This is similar to VLOOKUP or HLOOKUP in Excel.
If your mapping table contains the Source List and maps to a Target List-formatted Line Item or List-formatted Property, then use SUM in your formula. Setting your mapping table up this way says that each Target Item can map to many Source Items, so we are summing up all Source data that is to land in each Target List Item. This is similar to SUMIF in Excel.
Have a look at this article. It explains the differences and how to do it