I assume you are typing "test test______________" or similar (_ = space, this board strips concurrent spaces), and are questioning why spaces are stripped from commands but not regular input. If not, *shrug*.
mIRC strips the spaces so the $1 $2 $N parameters are properly filled in for commands. This behaviour has been discussed, ridiculed, tarred, feathered and what not many times before, you'll find a better explanation of the issue if you use the search feature.
Hope that helps.