XIRR: Different cash flows but same answer. Why?

I am using the xirr function to calculate the irr of two distinct cash flows streams over 360 months. The first 155 months of both streams are exactly the same, but the first stream has an additional 205 months that add some 12,242 of net additional cash! The XIRR of both streams is exactly the same (at least to 10 decimal places) but the NPV discounted at 10% is different!

This has to be mathematically impossible...Right?

I have attached the relevant excel worksheet and would appreciate someone pointing out the depths of my ignorance.