Unfortunately rates are not set in stone. Generally, there is one lump sum charge for the basic monthly services; anything else is in tiers. You are charged a base rate (which may increase unless your contract bars raising prices for several years); if you move to another a higher level of care, you pay another rate.
No one wants to deal with paying higher rates but a bump up is the reality when a family member has special dietary needs, becomes a fall risk, or when mental function or cognitive awareness and physical abilities decline.
You can also expect periodic increases from the facility which passes on labor and food hikes.