Comparisons vs Google Analytics reports usually have some level of discrepancy, but they nearly usually suggest that the page is structured in a way that favors either Google Analytics or Broadstreet reports, which is difficult to avoid.
For example, if a page invokes analytics earlier in the markup (toward the head of the page), it gets executed before any ads load. So in that case, a page view is tracked before ad views get tracked.
It’s not unusual that scripts toward the top of the page get executed more frequently than those later in the markup. The browser might hang on some external script, the user might stop the page from loading, or perhaps they’ve clicked on to a different page before the page is done loading.
So there are a few different reasons for discrepancies, but GA running before ads do (or after) is a very common one. Google also addresses that topic for DFP: https://support.google.co