Target activities are set at a page or a set of pages (template rules). Unique visitor data is counted on the pages activity is run. So, when crosschecking with reports from any analytics solution, the time period should match with Target Activity Report and the pages should also be chosen to match with those on which Target activity is running. Often times, a small variation between the two can lead to seeing a large variation in visitors being reported by two solutions; leading to significant time loss in figuring out cause of discrepancy. Different TimeZone settings in these solutions can also play a part in showing different sets of values
Do make sure to keep all these three aspects same when comparing reports.
*Having said that, the two solutions are different and so some amount of variation may be seen even when timezone, time and pages are same. There can be other reasons too like how two solutions define visitor etc but those are extra details. For most part, keeping the three in sync will help you do quick cross checking