<Tr> <Td> </Td> <Td> ↳ </Td> <Td> Phasmatodea (Phasmida) </Td> <Td> 2,500--3,300 </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> ↳ </Td> <Td> Phthiraptera </Td> <Td>> 3,000--~ 3,200 </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> ↳ </Td> <Td> Plecoptera </Td> <Td> 2,274 </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> ↳ </Td> <Td> Psocoptera </Td> <Td> 3,200--~ 3,500 </Td> <Td> </Td> </Tr>

Why is the range of rough estimate of number of living species so wide