If the data in both Analytics & Target match, then it could be a problem with Target not loading in certain situations. A few things that might be worth checking:
i) If you compare the 500k that visited the page vs the 100k that saw the Target activity, do you see any obvious differences? i.e., 30% of your 500k is Firefox but 2% of the 100k is
ii) If there's nothing indicating issues with specific devices/browsers etc, then you might want to set up something like this temporarily i.e., track an Analytics event when the request to Target succeeds, and then you can compare the number of visits to the test page with the number of visits that contained that "Target Success" Analytics event
Just to add, the other common issue is that the way Analytics & Target have been implemented means the A4T reporting integration doesn't work consistently; however, it doesn't sound like this is the problem given you see 100k reported in Target & 100k reported in Analytics