Could be due to a couple of things...
The wringer cap is connected to the spring tension, therefore it is designed to separate from the lower housing somewhat to allow for thick articles.
First - are you starting the item centered in the middle of the wringer? If not, that would be the correct procedure to ensure even pressure on the article.
Second - there is a possibility that the spring tension is tighter on one side than the other. This could cause the cap to raise on one side more readily than the other.