Why has SP1 for Server 2008 R2 left 4.5 GB of files in a randomly-named directory on my C drive?

Why has SP1 for Server 2008 R2 left 4.5 GB of files in a randomly-named
directory on my C drive?

We have been having a push to get all 2k8 R2 servers up to SP1, and I've
noticed that some of them have ~4.5 GB of files left in a folder in the
root of the system root (C: drive).
This directory has a seemingly random name, eg C:\1830d38bd5fd4ccdcf. It
contains the file "spinstall.exe", and another seemingly randomly named
folder, which contains various other subfolders for different
regions/languages (eg en-us, ja-jp, es-es) and an 879 MB
windows6.1-KB976932-X64.cab file.
It appears to be the files required in order to install the service pack.
I'm pretty sure it can be deleted with no ill effect, but I'd like to
understand what it is doing there in the first place.
Usually, when installing SP1, you would see two events from "Service Pack
Installer" in the eventlog. Event ID 1 would be logged when the
installation is started, and event ID 9 would be logged upon successful
completion of the SP install. On the servers which have this directory
left behind, event ID 1 is logged, but no event ID 9. The SP appears to
have been successfully installed, according to Winver.exe, and according
to event ID 6009 which is logged on the subsequent boot (which says
'Microsoft (R) Windows (R) 6.01. 7601 Service Pack 1 Multiprocessor
Free').
I have nothing in the PendingFileRenameOperations or SetupExecute registry
keys.