SOOMLA fetches data from the ad networks APIs and shows it on the dashboard so naturally the data should match the ad networks' reports. There are some cases where the numbers will be different though:
For most ad networks, SOOMLA only collects ad networks reports for the placements that it gets from SOOMLA SDK.
Let's say that you have placements A and B. Also, let's assume that SOOMLA SDK was only activated on a version of your app that shows ads with placement B. You won't see data for placement A.
On the SOOMLA dashboard, when you filter by a dimension that is not reported by the ad network you see SOOMLA calculations. Versions dimension doesn't exist on the ad networks reports so the numbers you see are probably smaller that what you expect.