"Less-expensive clamp meters use an average-detecting rectifier circuit that is then calibrated to read in RMS units; it is assumed in their design that the current is a sine wave of the local mains frequency, that is, either 50 or 60 Hz. If either of these assumptions are violated, incorrect readings may be obtained so when such meters are used with non-sinusoidal loads such as the ballasts used with fluorescent lamps or high-intensity discharge lamps or most modern computer and electronic equipment, the readings produced by such meters can be quite inaccurate."

Clamp meter - Wikipedia, the free encyclopedia

I'm not suggesting your clamp meter is one of the less expensive ones but it would explain your high readings if it was.

S :weedpoke:
synergist Reviewed by synergist on . Lights use almost double the wattage!!! This threw me into a loop, MY 250W HPS uses about 450Actual watts (about 4Amps @ 120volts) fully warmed up And my 400W HPS uses about 700Actual watts warmed up. This is killing me beause I designated 1 15Amp breaker (only use 70-80% of a breakers rated amp) for 2x 400W Lights, but now I can see that just one will almost max it out! With the fans and the one 400W HPS Im around 8.5Amps total. Rating: 5