I think it boils down to youth loan or not. With Bryn all youth loans must include a recall clause - hence they could farm him out elsewhere, however the Rory Donnelly loan deal that fell through was a season long loan with no recall clause, which meant he couldn't even play for Swansea U21s as a result...
I think the season-long deals are so that managers are able to realistically plan for the whole season without having to worry about losing the player at January after they have developed him into a world-beater!