Obviously transfermarkt is "just a bit of fun" anyway, but is there any logic at all to whether a transfer is recorded for the main club or the U21s?
For example Pedro Porro, who never made a start for City, gets recorded as a transfer out for the main club, while Romeo Lavia, who did play for City, is recorded as a sale for the U21s.
I'm guessing the reason is that Porro never played for the U21s, so was always recorded on their database as a full City player, while Lavia played for the first team, but was never a full time member of the squad, but it's all a bit odd.