Complex sentence: While designing the Pentium 4 platform in 1999/2000, the standard 20-pin ATX power connector was found insufficient to meet increasing power-line requirements; the standard was significantly revised into ATX12V 1.0 (ATX12V 1.x is sometimes inaccurately called ATX-P4).
Split into simpler sentences: While designing the Pentium 4 platform in 1999/2000, the standard 20-pin ATX power connector was deemed inadequate to supply increasing electrical load requirements. So, ATX was significantly revised into ATX12V 1.0 standard (that is why ATX12V 1.x is sometimes inaccurately called ATX-P4).