Why do the cpus get numbered: 0 1 2 4 ?

cpu0 at mainbus0: apid 0 (boot processor)
cpu0: Intel Pentium Pro (686-class), 198.97 MHz, id 0x619
cpu0: features fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR>
cpu0: features fbff<PGE,MCA,CMOV>
cpu0: I-cache 8 KB 32B/line 4-way, D-cache 8 KB 32B/line 2-way
cpu0: L2 cache 512 KB 32B/line 4-way
cpu0: ITLB 32 4 KB entries 4-way, 2 4 MB entries fully associative
cpu0: DTLB 64 4 KB entries 4-way, 8 4 MB entries 4-way
cpu0: calibrating local timer
cpu0: apic clock running at 66 MHz
cpu0: 32 page colors
cpu1 at mainbus0: apid 4 (application processor)
cpu1: starting
cpu1: Intel Pentium Pro (686-class), 198.95 MHz, id 0x619
cpu1: features fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR>
cpu1: features fbff<PGE,MCA,CMOV>
cpu1: I-cache 8 KB 32B/line 4-way, D-cache 8 KB 32B/line 2-way
cpu1: L2 cache 512 KB 32B/line 4-way
cpu1: ITLB 32 4 KB entries 4-way, 2 4 MB entries fully associative
cpu1: DTLB 64 4 KB entries 4-way, 8 4 MB entries 4-way
cpu2 at mainbus0: apid 1 (application processor)
cpu2: starting
cpu2: Intel Pentium Pro (686-class), 198.95 MHz, id 0x619
cpu2: features fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR>
cpu2: features fbff<PGE,MCA,CMOV>
cpu2: I-cache 8 KB 32B/line 4-way, D-cache 8 KB 32B/line 2-way
cpu2: L2 cache 512 KB 32B/line 4-way
cpu2: ITLB 32 4 KB entries 4-way, 2 4 MB entries fully associative
cpu2: DTLB 64 4 KB entries 4-way, 8 4 MB entries 4-way
cpu3 at mainbus0: apid 2 (application processor)
cpu3: starting
cpu3: Intel Pentium Pro (686-class), 198.95 MHz, id 0x619
cpu3: features fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR>
cpu3: features fbff<PGE,MCA,CMOV>
cpu3: I-cache 8 KB 32B/line 4-way, D-cache 8 KB 32B/line 2-way
cpu3: L2 cache 512 KB 32B/line 4-way
cpu3: ITLB 32 4 KB entries 4-way, 2 4 MB entries fully associative
cpu3: DTLB 64 4 KB entries 4-way, 8 4 MB entries 4-way

cpu2: CPU 1 running
cpu3: CPU 2 running
cpu1: CPU 4 running