What is Proposition 2 1/2 and why did my value increase more than 2 1/2 percent

Proposition 2 1/2 is the title given to an initiative petition adopted by voters of the Commonwealth of Massachusetts in 1980.
Its principal features are related to the total amount of property taxes that a city or town can raise each year. Other parts of the initiative, however, limited state agency assessments on cities and towns, prohibited unfunded state mandates and repealed school committee and binding arbitration for certain public employees. In addition, it reduced the motor vehicle excise tax and allowed renters a deduction on their state income tax.

This pamphlet provides further information on Proposition 2 1/2

The Massachusetts Department of Revenue has a YouTube video that also explains Proposition 2 1/2