Why (or how) do download speeds fluctuate?

How do download speeds fluctuate? it seems like you would have a signal coming from the router that gets to your device, but how is it communicated between the two devices how fast that information can be transmitted? in general, when I'm further away from the router, stuff downloads faster, which makes sense until I thought about it.
sure, there's interference just like with the radio, but how does your device sort that out? wouldn't it just download stuff incorrectly rather than slowly?
Sorry if this question doesn't make any sense. I'm just very confused and even if the answer is that I completely misunderstood what is going on, then I'd like to hear that.