While the Genesis block is timestamped 3rd of January, the hash of the Genesis block is astonishingly low. It has been theorized that Satoshi set himself a more challenging target for the Genesis block than strictly necessary and it might have taken some time until the block was found. It is also possible that he set the earlier date to match the headline of the Times which juxtaposed his goals so nicely.
Beyond that, the original software was designed not to start mining until it had found another peer on the network. Bitcoin 0.1 was only announced to the mailing list on the 8th of January at 19:27 UTC, and then the second node would have only appeared on the network once someone else’s interest was piqued enough to downloaded and run the software. Block 1 is timestamped 2009-01-09 2:54 UTC, 7 hours and 27 minutes after the email announcement.
While, Difficulty 1 was hard enough that it took until December 2009 for the difficulty to increase for the first time, it only took until 4:12 UTC for the next ten blocks (~7.8 min per block) to be published, so the gap between announcement and Block 1 is probably best explained by the second node only coming online shortly before Block 1.