transient "obsolete packages" with apt

On the testing and unstable systems that i run, i occasionally get a disturbing "there are N newly obsolete packages" message. For example

[0 root@squeak ~]# aptitude updateHit unstable Release.gpg [...]Get:42 unstable/main [2077kB]                          Fetched 15.2MB in 57s (262kB/s)                                                 Reading package lists... DoneUpdating debtags database...... DoneCurrent status: 3 updates [+3], 1136 new [-21212].There are 1804 newly obsolete packages.[255 root@squeak ~]# 

Usually, i can fix this with another aptitude update. Does this mean i've hit a mirror at an inopportune time? Or do i have apt mis-configured somehow?

Other people have documented this before, and casual conversation with friends lets me know it's not just me.

If this is due to some inconsistency at the mirrors (particularly for suites with rolling updates?), is there some way to engineer the mirror transfers so that this doesn't happen? For example (i have no idea how this stuff is done, so i'm just making this up):

  1. rsync the pool/ without --delete to all mirrors,
  2. rsync the Packages and Release, and DiffIndex files to all mirrors,
  3. rsync the pool/ with --delete

Or should apt itself deal with these circumstances differently? It seems like signed apt ought to be able to detect when something like this is amiss.

I'm reluctant to consider the situation acceptable, because i'd like to be able to trust apt in general. In particular, i can imagine apt eventually becoming more assertive about suggesting removal of newly obsolete and unsupported packages. If it is wrong about what's obsolete and unsupported, this could be dangerous.