CBBC is never tape-checked on an overnight basis as far as I'm aware. The double episode of TDG on 4 Jan was billed as running from 16:30 to 17:30, and the audience in this period was 576k (4.1%).
After 7 days BARB have had time to collect exact timings from the broadcasters, and so we now know it ran from 16:33:25 to 17:26:40. Tape-checking to the nearest minute gives you 16:33 to 17:27. For Live + VOSDAL we only have data in 5-minute blocks, so the figure used is the audience between 16:35 and 17:25, which was 593k/4.35%.
The consolidated figure of 625k/4.3% includes not only recordings viewed on days 2-8 but also an adjustment so that the figures are aligned with timings correct to the nearest minute (16:33 - 17:27). Looking at the breakdown, it won't have been a large adjustment on this occasion.
EDIT: Further analysis suggests the adjustment just mentioned deducted 3k:
576k (4.10%) - Live + VOSDAL (full slot)
593k (4.23%) - Live + VOSDAL (tape-checked to nearest 5 mins)
628k (4.35%) - Consolidated (tape-checked to nearest 5 mins)
625k (4.33%) - Consolidated (tape-checked to nearest 1 min)
This question has been bugging me a bit but I may as well ask. Are iPlayer viewings accounted for in the official rating? I ask that because the show does seem to be in the top 5/10 on iPlayer every week.