It will pop up all values for the time closest to the cursor that it actually has [a] value[s] for. So, if the points are on the same data source, there's a reasonable chance they'll have exactly the same time stamp (for polling data sources). But, if the data sources are different, it is unlikely the poll will align to the millisecond, and so you'll only have a popup on one or another series in the chart below the watchlist.
I suspect what's going on is that your points are logging on interval, and that the intervals are slightly misaligned. I think I see how this could happen. I think if you download the data you'll see the timestamps are ever so slightly different.