Report the spoken phrase, expected outcome, actual outcome, and what Siri transcribed on screen, using Apple’s feedback form.
Siri has a lot of quirks like that where the outcome doesn’t match what you could reasonably expect. This comes down to Siri’s natural language processing simply needing refining.
I use Siri heavily on Apple Watch, and I used to encounter similar issues, but they got fixed within days or weeks after I reported them.
One example would be: “At 9 a.m., remind me to do X.” Occasionally, Siri understands, “Add 9 a.m., remind me to do X.” Yet for the longest time she would go ahead and create a reminder with no due date, even though she clearly understood the time part of my request. A few days after I reported this issue, Siri started correctly tacking on a due date even when she understands “add” instead of “at.”