Agree it could be biofilm, also, when they do the culture and sensitivity on the urine, usually a result of >100,000 bacteria is "positive" and <100,000 are "negative" for most bacteria. Negative doesn't always mean no bacteria, just means usually not enough for a UTI. If any bacteria are present, but below the threshold, it can mean there is colonization in the bladder. It's not always a problem, but certain species of bacteria produce enzymes that raise the pH, breakdown wastes, and can precipitate out stones.
If struvite are a problem, and it keeps happening, sometimes it's worth treating without a positive UA to see if you have improvement. Being conservative tho helps prevent superbugs, so it should be only when other things are ruled out.