Where's the logic?

Ok, so here's the thing.
I decided to switch my usual hill repeats for 800s at Rotary Track this morning.
Wanted to let my legs recover fully from Sundays run. So off to the track, feeling decidedly average. 2km warm up, see how we go, if I blow up I'll call it a day and just go for a recovery trot. Well that was the plan. What happened was

3:01
3:02
2:59
3:00
3:00

So I ended up running nearly 5 seconds faster/rep than Thursday last week, with average speed 3:45/km and this two days after a 39km hilly bush run, where's the logic?
I was actually surprised when I hit the stopwatch on the first rep and never looked back.

4km warm down for 10km total.

A very pleasing but at the same time confusing session...