Because you are measuring the current at the wrong place? Or only outputing about 20W RMS. (RMS (or "continuous power") being the base meaningful rating for amps & speakers - forget peak power or any other power UNLESS they state exactly how those figures were derived.)
With a 30A fuse, that amp at 14.4V can put out a max of 30 x 14.4 = 432W RMS,
It's probably under a 300W RMS output when amp efficiency (~80%) and normal fuse loading (70-80%) is taken into account.
Your 8ohm speakers also means less output
At 2A, you might be outputting about 20W RMS,
To double that volume, you need 4x the current, hence 80W.