Their average only uses the the most recent polls (about 12 I think), so if a poll on the lower end replaces a higher poll, it can have a fairly big impact. All their polls are weighted equally.

FiveThirtyEight uses a more complex method with weightings based on both the quality and the age of the poll. They tend to be less volatile because of this, so don't sweat seemingly big jumps in the RCP average.

HRC probably has a lead of 6-8 points right now.

It's less political and more accurate. RCP is right wing and often try to skew things by removing good polls for Dems earlier than good ones for Reps.