Problem with variable incrementation via Script Assertion

Hi,

I added to TestSuite properties. One of them is ID variable. 



i want to increment my property by 1. I add in Script assertion code but i dont know why my variable increased by 2. 



I have few screens from soapUI.



So i show you step by step what is happen.

ID property exuals 1.

Start request with Script assertion and on the beggining variable equals 2 and after do the line ID = ID + 1 is 3. Why?





Can somebody help me?