Star Wars: Rogue One - 11 Biggest Unanswered Questions
9. How Did R2-D2 & C-3PO Get On The Tantive IV?
The Question: Considering that C-3PO can be heard complaining about the rebel fleet taking off for Scarif without him, how did he and R2-D2 even make it onto the Tantive IV in time for A New Hope?
The Likely Answer: The real answer is "because fan service", though the movie's official novelisation actually explains that the pair just about made it onto one of the very final ships heading to Scarif (as the ships seen flying off are X-Wings rather than the main bulk of the fleet).
It's definitely awkward editing-wise and could've been executed much more smoothly, but there's enough vaguely implied downtime for the two to end up in the fleet.