apt{-cache,-get,itude} show wrong version of package after update

[Jean-Baptiste Thomas <cau2jeaf1honoq@laposte.net>]

After apt-get update, attempting to install ntp tries to
download version 1:4.2.8p10+dfsg-3+deb9u1 and fails. It tries
to download +deb9u1 because

  $ aptitude show ntp
  Package: ntp
  Version: 1:4.2.8p10+dfsg-3+deb9u1
  State: not installed
  [...]

and it fails because the version of the package in the Debian 9
mirror listed in /etc/apt/sources.list is +deb9u2 :

  ntp_4.2.8p10+dfsg-3+deb9u2_amd64.deb

I don't understand what went wrong. apt-get update seemed to go
well, only complaining about missing "DEP-11 64x64 Icons", which
are presumably not a vital part of ntp.

How is this possible ? I'm confused.