Number of sessions changing
I've been having some issues with Google Analytics as I monitor a couple of sites. I'm having some problems with the number of sessions changing when I apply a filter, and the number of sessions changing from day to day.
Re: Number of sessions changing
1: When you are applying your segment, you could be subject to sampling so look for the sampling indicator to see if that's the case. When sampled all data is impacted.
2. Intra-day processing happens all day until and if you hit the data limits. After there is a final nightly process that runs so you can see some variances when looking at traffic for today and then looking at it a day later. (If you were doing that.)
Hope that helps.