So it's not so far out of the ordinary to be unexpected?
In my experience, those sorts of variances are pretty common with lifting and endurance running. Especially for non-professional level athletes.
In Climbing, I know that when I'm on form I can generally get a route graded 7b+ after a couple hours of working it and a few redpoint attempts. The few 7c routes I've done each took about a dozen days of work spread out over the course of a month.
The route I did on one of my pre-cold days was rated 8a+ (though it was likely a bit overgraded), and I got it second go. Like I said, I came back a week later, and I couldn't even do the single hardest move in isolation.
So, that far out of the ordinary.
But I suspect it wasn't the cold coming on that did for me, on the one hand, more likely exposure to somebody with a new and improved bug a day or so before. And on the other hand, I'm not sure that co-workers who got to listen to me cough would have considered the run worth it.
I climb rocks, and a few times I've had days where, for no reason I could explain, I have performed far above my normal abilities (1). Big days, where I've sent routes that were way above my head. Routes that I couldn't pull individual moves on when I came back to them a week later.
Then the next day I would be clobbered by a cold so violent that I'd be bedridden for an entire day.
I chalk it up to the body knowing that a cold is on the way and storing up all the immunities, defences, and reserve energy that it will need to fight it. If you time it right, you can steal all that stuff and channel it into one day of hard climbing. Of course then when the cold does come, the body has nothing to fight it with so you get crushed.
Considering how good it feels to be that on form, even just for a single session, I think it's actually worth it.
(1) If you climb, you'll know that it's a very measurable sport. If you can boulder a certain grade, you can get on a given problem and have a reasonable expectation of being able to work it out. Or alternatively, you can know for a fact that you could train on this one particular problem for an entire year and never top out. So when you're having a day like the one I describe above, it's the equivalent of showing up at the gym one day and finding you can suddenly bench press 50 pounds more than yesterday.