HSPICE node naming convention
From the hspice perspective and how it simulate circuit BIT0:5 and BIT0:6 are separate nodes. 

Regarding the naming convention extraction program try to keep the name of the nodes the same as in schematic(for circuit designer convenience). However sometimes it is not possible -> for example if using RC extraction one metal path will be represented by several cascaded circuits like so:

BIT0       BIT0:1        BIT0:2         BIT0:3
    R1             R2             R3
---/\/\/\----|----/\/\/\----|----/\/\/\---
            ---            ---
       C1   ---         C2 ---
             |              |
            ___            ___


So this naming convention is just a mark that those nets correspond somewhat to schematic net.