Scrims generally improve your gameplay as a whole because you're facing people around your own skill level so it's natural your aim also gets better. On ETL1 the usual 20v20s are big clusterfucks, you get killed by random nades and 3v1s half the time so it's not really an ideal place to practice.
That said, the 45% accuracy comes with a ton of practice. And to answer your question: they crouch so it's harder for you to kill them.