This is because you are selecting SQL tables which are containing memory and logical disks values by default in (Bytes), therefore you have to add additional SQL Queries to convert Bytes in MB or GB.
As example: SELECT * FROM Inv_HW_Physical_Memory --// Memory is in Bytes by default
But if you will check "Memory Configuration by Computer" report, then you will see there that memory capacity is converted from bytes to MB, since SQL Query calculates it in MB
Just complete creation of required report and then add additional SQL queries to calculates Bytes in MB or GB.
Thanks,
IP.