Wire isn't just wire.. there are variations between brands, and even within brands.
Sure, the wire itself is JUST wire, but the purity of the copper is different from brand to brand... and as you've stated Chauncey, the strand count matters.. you also need to take into account the temperature in which the wire can be run in. The temperature isn't generally a problem, though.
If you have a cheap wire from an unknown brand, it might be decent at best.. but it's most likely 60% or more plastic shielding.. which is GREAT if you want to protect it from the heat, but most likely you've purchased a 0/2awg wire with a stamp that says 1/0 :\