Forecast summaries

I have a potential in my system which closed in May 2015. when I look at the summary of May it does show as 0 . But if i click on the 0 than it will show the win closed potential behind the 0 of 12,000 USD. In an other occasion i have moved potentials to another later date. In the summary they still show as pipeline USD 142,000 but if i click in the USD 142,000,- is shows no details. In another occasion i look at 1,200,000 in 12 different potentials, but in the summary it shows only USD 583,000,- but if i click on the 583,000 the details are showing 1,200,000.- It seems to me that normally the details behind the figures are correct but the summary is giving wrong figures. How is this possible?