[Yum] ok, so how come "yum update" doesn't?

[Robert P. J. Day]

  on a test system, i ran "yum list updates" and got 400+ packages for
which there are updated versions in the repos.  (as a quick example, at
the end of the list, i verified that i have ypbind-1.12-2 installed and
ypbind-1.12.3 in the repository.  so far, so good.)

  so i run

  # yum update

and i get a slew of messages of the form 

[update: <package> <version>]
...
Is this ok [y/N]: 

  i type "y", hit return, looooooooong pause with much disk activity, then

"Calculating available disk space - this could take a bit"

... another long pause and more disk activity, then numerous lines of
output:

installing package <blah> needs xxyyMB on the /usr filesystem

where the numeric value "xxyy" is clearly increasing slowly and finishes
at 286MB.  there's about 525M available on /usr, and i don't get an error
message but when it's done, nothing's been updated, nothing in the 
log file /var/log/yum.log.  am i missing something?  why was nothing
updated?  running "yum list updates" again shows the same 400+ packages
that are available for updating.

thoughts?

rday