That site, like Zillow and some others, uses algorithms to attempt to determine value. First, understand that the house could well be worth less than $68,616 or more than $74,500. They're really not very accurate. So: Pay no attention to the numbers.
To answer your specific question, it's likely that the numbers that get pumped into the algorithm have changed. The calculations take into consideration what it considers comps or recent sales. So if a property sold nearby for, say, $85,000, it would boost the number for the house you're looking at upward. Or maybe it was using an old comp at, perhaps, $60,000. But now the comp is more than 6 months old so it's dropped out of the calculation, raising the average.
Those algorithms also take into account price moves in your county and, to a lesser degree, in your state. So maybe nothing changed with the house or the neighborhood. But prices rose somewhere else in Philadelphia County.
Those algorithms also consider tax assessments. I don't know when your area comes out with them, but those can exert an upward or downward force on the calculations.
So that's why the number changed. Does it mean that the house actually rose in value? Probably not. Does it mean that the house is now worth $74,500. Absolutely not. Have a Realtor run a CMA for you on the property to get a real idea of the value of the house.
Hope that helps.