Small nit, and this doesn't detract from your points. I don't think this is universally true by definition, even if it is almost always true. You could come up with some rare conditions where your traffic at t+5 minutes is actually easier to predict than at t+20 seconds. Of course, even in that case you're better off (ceteris paribus) being able to spin things up in 20 seconds.
[0] For example I can tell you exactly when spikes will happen to Netflix's servers on Saturday morning (because the kids all get up at the same time). And I can tell you there will be spikes on the hour during prime time as people shift from linear TV to streaming (or at least they did a lot more 10 years ago!). I can also tell you when spikes to Alexa will be because I already know what times peoples alarms are set for.
For example, things rarely start before they're scheduled, so if you know something is supposed to happen at 5 and last an hour, it's reasonable to assume it will be happening at 5:10, so if it's 4:59 right now, you're more certain that it will happen in 11 minutes than in one minute.
Of course, as time progresses your uncertainty curve flattens out (at 5:09 you're pretty certain it'll happen any moment now), but that's only in the future.
Even then, it's quite reasonable to say that there's no time leading up to the event where you're more confident it will start imminently than you're confident it will be occurring further out.