232-245 actually. Blizzard, since Ulduar, have tried to balance raids and ilvls so that groups can progress into the next raid tier equivalent content. Nowadays on Live, it's T19 normal -> T20 normal and T19 heroic -> T20 heroic. But for ToC -> ICC design, it was 10-man -> 10-man and 25-man -> 25man. This was largely because you had 10-man guilds and 25-man guilds. Forcing a 25-man guild to run/farm 10-man raids doesn't work well. 258 ilvl ToGC gear is from the 25-man raid, which exceeds the ilvl of ICC10 normal, but is only 6 ilvl under ICC25 normal. Blizzard's philosophy has always been that 5-6 ilvl is what they consider to be "the sweet spot" for an ilvl upgrade that actually feels like an upgrade.
Weren't in full epics? Are you talking about private servers or how Blizzard designed and tuned the raid? If we're talking about Blizzard's design, it most definitely was designed for people in full epics. As for a reason for 258 ilvl geared players to do 10 normal - you have to have someone who has defeated the Lich King on normal before you can access heroic.