If you add up the Surveillance, Retrieval, Speed, and Range stats for each of the new sub parts, it will equal 105 points. However, the Favor stat is unrestricted by this limitation, i.e., some parts have more favor, some less, such that the sum total amount of stats on each part are different if you include Favor. Therefore, it makes sense to prefer parts that have a lot of favor since that would make your sub's overall stat total better.