I tried variable length pauses when testing this. It still just doesn't understand how to parse, let alone tone and context, like at all.So, Siri is as intelligent as a human when listening to your requests on a phone call. As noted by other forum posters, it would be more precise to articulate, "find the average of 19, (AND) 17, and 15," or you could opt to type to Siri if you do a lot of calculations like that in natural language.
Also, did you enable any of the Siri accessibility options? Specifically, the setting that adjusts the pause time? It could be why Siri thinks you need extra time between words.
My comments here are not meant to deny or dismiss your feelings about Siri being awful; I’m simply offering a few suggestions.
Again, typing the info into a Siri text box negates the need for Siri, I'd rather use Calculator or just a simple Google search because then at least I can copy/paste the answer.
Siri is barely better than it was in 2015 at understanding and parsing your verbal queries.