I'm a windows guy also. This is about using Unraid, not the underlying linux OS.
The split level setting is about a user share, not about a drive. In fact, the whole point of split level is to keep certain files together so they don't go to other drives.
For example, suppose you have a music collection, and you are old-school and like to listen to "albums". You are playing an album, and the first several songs are all on the same disk. So it plays the songs from that disk, but since the other disks aren't being used, they are spun down.
Now it gets ready to play the next song from the album, but that song has been allowed to be split to another disk. Since that other disk is spun down, you have to wait for it to spin up before the song can begin playing.
Same principle when binge-watching TV series. Some people will prefer to keep all the files for a specific TV series on the same disk so there is no spinup delay between episodes.
I don't care about split level, mostly because I have my whole music collection restricted to a single disk anyway, and I don't care about TV much. So for most user shares, I just set split level to allow splitting anything.
You have split level set so it WON'T split anything. Instead, you have told it that you will manually create new folders on other disks when you want files to go to other disks.
You can click on the label for any setting in the webUI to toggle help for that setting.
For simplicity, try changing it to "Automatically split any directory as required".