Expense differences?? EV settings??

Since a server update all of the projects had almost doubled thier expense value, when i looked in to this the exchange rate set for British Pound had gone to 1.00000 the same as the base rate dollor currency.

I have reset the British Pound to .501762 and the majority of the projects have gone back to thier original expense cost.

I do have an issue with some others though as they do not match, I looked at the WBS EV settings within these projects and noticed that they have the setting of PF=1/(Cost Performance Index*Schedule Performance Index)

Is this the reason why this is making my expenses wrong or am I getting this totally wrong? what steps would I take to bring the expenses back in line without going through all the projects and manually changing them?

Thanks

John