G-RAID transfer speeds 28TB vs 12TB - sequence files

I’ve been using some G-RAID 28TB drives for backing up a large amount of 7MB sequence files and have noticed that rather then reaching the advertised 510MB/s transfer speeds, i’m only getting around 130MB/s. If instead I transfer a large single file I can get 230MB/s. The drive is at 80% capacity now so this was closer to 510MB/s when empty, however when transferring sequence files the transfer speeds have stayed fairly consistent.

I’ve since started using some G-RAID 12TB drives for the same task and am getting much better speeds for sequence files, around 270MB/s, even though the advertised transfer speed is slower at 440MB/s. Why would this be?