Why does the data in GA4 not match Universal Analytics completely?

GA4 measures sessions differently now, e.g. new campaign parameters and active sessions at midnight previously started a new session. This is no longer the case. In addition, GA4 does not currently allow for filtering traffic so if you previously filtered out hotel Wifi or office IPs, this traffic will now be included in GA4. We will update the filters as needed when and if they become available in GA4 in the future.
For more information, please visit the Google Analytics Help Center
Additionally, GA4 uses a different attribution model. Universal Analytics used last non-direct click. This model attributed 100% of the credit for a conversion to the last channel which brought a user to the site. If the last channel was direct, then the previous channel was credited, unless direct was the only channel. GA4 is using a data-driven attribution model which is unique to each account and each conversion event. Attribution uses machine learning algorithms to evaluate both converting and non-converting paths. The resulting Data-driven model learns how different touchpoints impact conversion outcomes. It then decides which channels are most likely to drive conversions and attributes credit to these channels based on their likelihood. 
For more information, please visit the Google Analytics Help Center article on attribution