I dont know how its decided against what kind of players I get put into but, all the BGs I que in to, I get mached with guys who have pvp gearscore 5.5k and up, while mine is barely 4k, making it fairly usless for me to try anything to help the team.
Yes, you will get that when the lower geared queue takes a long time. If you don't get matched with your lower tier in a certain amount of time, you get upgraded to the higher tier.
It's only likely to happen when population is low, otherwise it should be pretty fast.