ADNS-9800 optical mouse sensor mapping out a parallelogram instead of rectangle

I've received an ADNS-9800 arduino optical sensor for usage in measuring the position of my device. The specific component can be found here:

I've managed to get the thing set up and can read values from its registers. The problem is, is that the system seems to be mapping out a parallelogram instead of a rectangle. Here's an example image:

I just can't seem to work out what could be causing this. I wonder if I'm not reading over the serial port correctly. The Delta X and Delta Y values are comprised of two registers each, LSB and MSB. I'm pretty sure these are being handled correctly.

Any ideas guys?

Thank you

Hello
I'm trying to do the same project as you.

I also bought the same component:

BUt I only managed to have -1 for X and Y in debug.

did you have to change the code a lot to have it work?