Segmentation: The conditions defined in the RTP segments do not cover all the website traffic.
Tag implementation: The RTP or the GA tag is not installed on all the website pages
Tag Timing: The data is sent from RTP to GA once both tags are loaded on the page. In some cases, visitors are leaving the page before the data sent to GA. The recommendation here is to locate both the RTP and the GA tag higher in the code so they will load faster.
Web Bot: RTP is treating web bots and crawlers differently than how GA does. There are cases where RTP identifies these "fake" user and doesn't show campaigns / send data to GA while the visit will be recorded in GA as all other sessions.
Session Duration: A session in RTP last for 30 minutes from the last click, while in GA this can be customized (see Session and campaign timeout handling). This can cause some gaps in the numbers user sees.
Sampling: GA shows sample of the data and not 100% accurate numbers and metrics, see: