Data from the Mauna Loa Observatory (that is compiled by NOAA), while it shows an ever-increasing trend from the late 1950s, had the C02 reading around 380ppm in 2006. Assuming this is legit data, how could the number be 455ppm in 2005?

Because that number is just for CO2. The 455 number includes other agents that act like CO2. Sort of like saying that a guy got hurt being hit by a thin piece of fiberglass, without noting that there was a two ton car propelling that fiberglass bumper forward.