You need to let your tap water gas off CO2 before testing pH to get an accurate reading. Put tap water in a cup, leave overnight, then test the next day. See if that matches your tank water.
Other than that the only other explanation is that you have something in the tank adding carbonate which raises pH. Any new rocks etc been added recently?
Do you have an old aqua soil? They lose their buffering capacity over time.
Last edited by Straight shooter; 11-28-2015 at 01:36 PM.