It looks like a pre-modded computer supply. Big tip-off is the empty grommet where all the wires would normally be routed:
For another thing, most computer supplies put out exactly 12v. Once you load it down with high current, the output voltage drops, possibly below what the charger wants to see. If you mod your own, you can sometimes increase the output voltage to at least 13v. Not only does this give you a little extra cushion for voltage drop, the higher voltage also means the charger will draw less current (which reduces the voltage drop).