Results from Select with values out of range

Hi,

 

I have a Table "Cuentas_Maestro" with Varchar type field "Numero_cuenta"  i need to get all of records inside a specific range but i get records out of that range "in yellow", why??

 

image:

 

 

What i have to do?

 

I have tried with:

 

ISNUMERIC,CONVERT AND CAST Clause but i don´t have the correct result 

 

for example:

 

select * from Cuentas_Maestro

where isnumeric(Cuentas_Maestro.Numero_cuenta) between 50000000 and 89999999

 

and nothing...