4 days ago, I spent most of the day mining. I kept going until I hit surplus SP (my SP turned yellow).

When I starting mining again last night, I was no longer fatigued and my SP was again white. I mined for a couple of hours and got 20,000 SP. Then I started mining again this morning. After 3,000 SP, I am now fatigued again. Not gathering fatigue... but actual surplus SP.

I thought it took about 80k to hit surplus. Why am I hitting it after 23k on miner?