Why am I measuring the power different than the given rated power?
The Buerkert solenoid valve is listed in RS as having an RS part number of 337-5521 and, on that page there is a data sheet that tells you that the holding power is 8 watts and this means you can relax the drive voltage from 24 volts to something a bit lower and keep the solenoid operated.

So, assuming 55 ohms is an accurate measurement of resistance, the voltage required to achieve 8 watts is more like 21 volts. If you maintained operation at 24 volts then the power would be the 10.5 watts you calculate.