Analytics do tend to be faulty at times, so I'd recommend (without overcomplicating things too much) having more than one platform for tracking your analytics, if possible. I use both HubSpot's analytics tools and Salesforce reporting, and often compare the two sets of results to check them against each other. Generally, though, you should be able to spot drastic changes in reporting and know that it was either likely caused by a big shift in your marketing efforts, or else there may be a bug in the system. Always double check that you set up your tracking tokens correctly too.