Weird FI (Little Upsilon)

So, my profit has actually been increasing steadily, and my costs decreasing. This month I had:

Total Income and Cost 601,538.84M SC$ Click to see history 98,898.42M SC$ Click to see history
Profit / Shortage 502,640.42M SC$ Click to see history


Yet my FI has plummeted from 350 down to 165.

I don't really care about rank or score; it's the money that matters. Still, I find this a little weird. Any thoughts?

I found that when my costs went below 100B my FI tanked. I think perhaps there is a different formula for calculating the FI with costs under and over 100B.