Thanks all for help. Now that you've educated me, I suspect that some server disconnects I suffered between accepting a 2/day daily at lvl 50 and levelling to 51 may have thrown things out of joint. And now that I know that I don't have to finish all the foundry quests the same day, I'm a lot happier.
Hmm, that doesn't match my experience. Accepted a 2/day daily at lvl 50, leveled to 51 inside the first quest, completed it, then returned to Protector's Enclave to find that the daily that I had accepted had changed to 4/day behind my back. Hence my surprise.