ComboBox with Binded width property wider than it should be????

Hello,

I need the width of dropdown of the combobox menu to be set relative to Combobox (toggle button) width. The reason is that I need to control the laoyut of elements inside the ComboBoxItem. The problem is if I databind the width of a grid which is inside the Comboxitem then the witdth gets some aditional pixels that should not be there and the reason for it is unknown. I did a prototype where I removed all properties to make sure what is the problem. As a result, I found the if I databind the width property than the element gets bigger on about 6px wider than supposed to be. In case I don't bind the property then it works correctly, however, I cannot control layout I was planned. Please let me if there is solution for this problem but without using margins. I am wondering if anybody knows a reason for it. Thanks you in advance.