You have to understand that Noncalvinists have a very small view of the Creator, and a bigger view of His creation. They actually think of God as existing in something bigger and independent of Himself, like the rules of math. It is impossible for them to see it otherwise.
However, it is impossible for God to lie. He would have to deny Himself to do that.
It is impossible because He is Holy, and could thus never desire to do it...not because He lacks the power.