There are folks from the Defender group at Microsoft who read these groups,
so your question will get read.
I seem to recall that there was an issue with a specific set of definitions
awhile back. I've been away on vacation, so my memory about the details is
hazy--I may be completely off base.
What you need to look for is a) whether a visit to WU/MU gets you the
update, and b) whether the next time there is an update pushed to
everyone--I think you can see this by posts in the announcements group
here--you get it within a reasonable amount of time.
Now--as to the absolute definition level that is current:
The most current set of definitions will always be found at
www.microsoft.com/security/portal Look in the right hand column for 32-bit
Windows Defender definitions.
The version I was reading back to you was taken from my Windows Home Server,
which updates rather conservatively--probably no more than once a day--I
really don't expect Defender to ever see anything on that box. Definitions
at the security portal may change a number of times a day. Definitions sent
via WU/MU used to be rather infrequent--perhaps every 2 weeks. I have no
tracked their frequency lately, but I'd be surprised if the average was more
frequent than weekly.
There's a significant resource requirement--both in terms of server and
distribution channel bandwidth, and in terms of CPU usage on client
machines--to distributing updates. I'm willing to trust Microsoft to get
definitions out quickly when there is an issue that requires it--and when
I'm feeling paranoid, or about to scan a new machine where I have some
suspicions, I can go to the security portal and get the latest.
So--I don't think there's a systematic problem--but if mine doesn't catch up
with or exceed yours by tomorrow, I could be convinced!