Hi, Bob.
This behavior is due to a route change. If multiple routers respond at the next-to-the-last hop, then some sub-set number of samples were sent to that particular router (because some other router received the other ones).
As you scroll back in time, PingPlotter always tries to pick a route that was active in the time period you're focused on, so when you did this for some time in the past, the *other* router was shown. That's why you have two different numbers here. At sometime 533 samples before the current time, this hop had another router servicing it.
There are a few ways of handling this.
First, if you understand what's happening, the data representation might match your needs. You can see the routes that participated during this sample period in the "Route Changes" panel on the left. Selecting a different time in that area will show the route as of that time (and doing a "Copy as ..." will use the data that was sent to the routers participating in that route.)
If you find that you don't care about the change to the router at that hop, you can combine this data by right-clicking on the hop in question and selecting the "Add route change mask" option, which will treat these two routers as a single router in the future, and will also merge that data for these two routers together in the data you're currently reviewing. Doing a "Copy as ..." after this should show you results a bit more consistent with your previous expectations.
Let us know if this makes sense!
- Pete