Simple answer - it's not Matt LeBlanc under the helmet. Because the actor was also busy filming the TV series "Friends" at the same time as making this movie, a double was used for this sequence to allow for some flexibility in the actor's schedule. One could tell it was not Matt LeBlanc because the acting was far more believable.
Also, because he's a military officer, his helmet is more advanced and protective than the ones used by the Robinsons.
Another possible answer would be toys. The Major Don West action figure from Trendmasters had the helmet, a rifle, and an alien bug as accessories.
