apt-pinning, strange behavior

[berenger.morel@neutralite.org]

Since I had to reinstall from my last kernel error, I decided to stay 
with stable on that computer, but I need some softwares in less outdated 
versions, like development libraries or i3 ( this one is not a need but 
a question of comfort, I admit ), so I want to use apt-pining.

I have set all packages from stable to a priority of 900 and testing 
packages with 500.
But tzdata wants to upgrade, for an unknown reason. Explicitly making 
it to a priority of 900 for stable fixes that, but I can not understand 
why it is needed?