Shops updating
 

Is whether or not a shop will update with new troops, spells and items random?

I thought it was related to completing the quest given at that shop that it would gain new stuff, but this doesn't seem to always be the case.

In my warrior's game for instance several shops got better after the quest was done, that don't change at all for my mage. (example, the magic school gained inquistors and new scrolls after I brought back the chronicles)

After some quests some of the shops/castles get more/better army but the random shops all over the game are finished once you buy all the stuff from them.