Breast- "isodense nodular density.. mlo and ml views slightly more prominent than 2020 but a diff technique was used" birad 3. my 2020 exam never mentioned this (done at a diff facility). said 6 mo follow up. ultrasound clear. what could it be?

It is probably nothing and they are just being careful. Radiologists vary in how aggressively they read the films, and what they are looking at was less obvious two years ago, maybe because it was actually less obvious, maybe because of different techniques. In any case the change has been minimal and if they were really concerned they would have a different recommendation.