The problem for the Japanese in all of this is that their expectations of how the Pacific war would be fought was based on their capacity for and doctrines of combat. That is, they had no clue that the US would send as an initial landing force over 20,000 troops to Guadalcanal. Instead, their thinking was that this was another Mankin Atoll raid. In their view the later incident was within their expected norms of warfare, Guadalcanal wasn't.
Hence, when the Japanese first responded to Guadalcanal it was with the expectation of meeting a small US force of say, no more than 1000 men at most. This explains why they sent just the 900 man Ickihi Detachment along with a few cruisers and destroyers to soften up the island a bit. It simply was beyond their own thinking that the US would have landed such a large force and would be able to supply it.

The same thinking went into many of their island defense systems prior to Mankin Atoll. The Japanese never thought the US would be capable of actually landing on and taking a defended island. Again, they based their assumptions on their own amphibious capacity. They assumed the US, as they themselves had at Wake, would be landing small units with limited support against the defenders.
Instead, they got the shock of Guadalcanal and Tarawa where the US showed up with massive firepower and overwhelming numbers.