I've found that Active, which is the default, is pretty useless as it tends to be a self-fulfilling prophecy sort of thing. Everyone sees the most active stuff and they become more active, keeping them at the top.
Hot works better for a balance between New and Active.
I recommend changing your default sort in your settings to anything but Active. While you are there, might want to change the default grouping from All to Subscribed.
Oddly enough, Active has been the best for me. When I sort by Hot, I get almost entirely 2+ year old posts. Guess it all depends what communities you've joined
Hot is good but it works more like "rising" on reddit did. I sort of go between hot and top 6/12 hour for a combo of rising things and things that have a lot of interaction but aren't stale.
Active isn't very good. I've been having good results with hot/new. Haven't tried top by time yet.
Then I would suppose same community might just be a function of traffic those are receiving vs others so your odds are higher to see the same. Should smooth over as more places get better established.
Because the default sorting algorithms for both posts and comments from reddit are not implement. Imo quite annoying because of issues like the one you mentioned. But not a lot of people seem to care.