Need some assistance with making a battery box
The battery has some internal resistance, so when you draw current from it, the output voltage will drop - how much drop will depend on the load current.

The small wires you appear to be using may also have significant resistance, and again will cause some voltage drop.  Likewise, there will be a small voltage drop across the fuses.

I wouldn't be surprised by a few tenth's of a volt drop with some load.