Ah, that's an interesting and (to me at least) non-obvious fact. Really, there's a lot about forts that is obscure... Like the fact that a fort with a high defense doesn't necessarily fire more arrows than a fort with low defense, if someone tries to storm it.
Originally posted by LintMan: The castle/tower/etc supply rating is only used for supplying your troops during seiges. That value
does not actually affect the normal everyday supply in that or the surrounding.
The quoted thread has a formula that says that it only depends on absolute admin, i.e. 20Admin = +80supply in the fortified province. This means it is even independent on the popluation, etc. Is that really true? Sorry, I must go and change my pretender-design then asap...