We typically had a wider gap allowed for low-ranked matches because there were often fewer players at that level queuing and not having it sometimes meant people get hard stuck with almost nobody to queue for down there. There was also a fairly wide skill gap at that level compared to how tight the high ranks are. It's possible now that so many people are in the ladder we can lower this gap a bit more.
I have lowered this gap a bit, but this may come at the cost of getting fewer matches.