They're not going to be skewed. HGST disks are usually more expensive, so that limits the quantity that they buy. The seagate disks are usually cheaper but have a slightly higher (except when it's a brand new line) failure rate. When filling out a single server I go with the HGST disks because the premium price and quality means fewer failures, but it's more cost effective to go for lots of seagate disks when you have more redundancy and can eat more failures.