how can low watt tube amps drive speakers with higher power requirements

"Back in the day" we bought stereo systems and the first thing asked was, " How many watts?" Every manufacturer played the numbers game to perfection because we had become "spec sheet scientists". {RMS @ 8 Ohms 20 - 20000...THD 0.000001 S/N ratio. Wow and Flutter. You know the drill} Whatever....we used to kick car tires too. Tonight my kit blasts out a pathetic 20 watts per channel. It runs fairly high distortion levels relative to most. I only have two channels. Two speakers. No sub. One turntable. One CD player. Simple, endlessly astounding, intimate, a sound stage to die for, with totally lifelike audio reproduction, (IMO) I live in my happy place. Why did I take such a long route to achieve such an elusive yet rewarding destination? I think because I wasn't "listening" , I was measuring, counting, and competing. Just sayin' (Wish someone had told me that a long time ago)

No you are not missing something.
Any amp will work. It’s just a matter of how well and the results will vary widely.

The main thing always is to avoid clipping. That is public enemy #1.
https://en.m.wikipedia.org/wiki/Clipping_(signal_processing)
Tube amps typically soft clip as do some others.  You seemingly get away with fewer watts when soft clipping but the result is distortion nonetheless.

My 91db speakers are driven just fine by my 20W tube amp.  (I will say, though, that with a similarly-priced SS amp at 40W, they sounded like sh*t.). I doubt that there's a general rule that you could follow that would trump the judgment:  "What does it sound like?  Do I like what it sounds like?"

If you buy a lot of equipment over the years you will find out that consumer grade receivers at 100 wpc have trouble keeping up with 25 wpc tube amps. You'll also find some 60 watt amps that kill 100 wpc receivers. The difference is power supplies. Good power supplies provide great experiences. The best way to stay on course is keep away from consumer electronics and buy quality audio from know good specialty manufacturers. 

The other things that make a difference in regards to how much power is needed is how loud will you go and how far away from the speakers will you be.