You have to regenerate the variable's value before use, as they're designed to retain the last value you put into it.
You can reduce your command(provided it's the one you posted before) to this:
Press down NumPad Add key
Start Loop : Repeat 13 Times
Press down NumPad 7 key
Set decimal [Dance Time] value as random from 0,4 to 0,8
Pause a variable number of seconds [dance time]
Release NumPad 7 key
Press down NumPad 1 key
Set decimal [Dance Time] value as random from 0,4 to 0,8
Pause a variable number of seconds [Dance Time]
Release NumPad 1 key
Pause 1,5 seconds
End Loop
Release NumPad Add key