On page 61 of the rulebook it states that raising levies has the potential to lower the province level.

"Each time a levy is raised, the province is reduced by one level until the levy disbands. A number of militia units up the province's base rating can be created, so a level 3 province can must three companies; this will reduce the province level to 2. ( in the next turn two levies could be raised reducing the level to 1.) The province level remains reduced until the entire levy disbands. If all the militia units are destroyed in battle outside their home province, the province never recovers. (A militia unit defeated on its home ground simply disbands and the province level is restored.)"