Optimization Window, data mismatch
One common source of a data mismatch is Snapchats Optimization Window, which comes in two flavours. Either it is a 28-day click, 1-day view or a 7-day click, 0-day view. By default, Snapchat uses the 28/1 option. If another Optimization Window is needed it is possible to change in the Snapchat Manage ads table.
Note that those two settings correspond to Funnels "Swipe Attribution" and "View-through Attribution" in the connect flow. So if the Snapchat UI is using a 28/1 "Optimization window", the corresponding setting when connecting a source in Funnel should be 28d Swipe, 1d view-through.
Be aware that Funnel includes deleted ads in the API requests, while customers can get deleted ads in the Snapchat UI at the campaign level, but at the ad level, they won't be able to see it once deleted.
Note that Funnel currently refreshes data that are up to 50 days old. After that, the data will not be updated again. This means that if a status (like Campaign Status) is changed in Snapchat, the change will not be reflected in Funnel for data older than 50 days.
An easy way to see when the data was last updated is to use the "Data downloaded at" field that gives this information.