The first is arguably just a vanilla launcher with the one shot already loaded and no ammo bin attached to the loader (the 0.5 tons comes from rounding up the tonnage of that shot).
The improved version probably removes the loader, and the rest of the tonnage probably comes in the form of fire control gear, extra armor to protect the weapon from enemy fire, and what not. Rocket Launchers which notably DON'T have fire control gear (or loaders for that matter) get very very light since they're not spending tonnage on such luxuries.
Oh, trust me, I'm fully looking at rocket launchers (which are close enough for government work to being the
real one-shot MRMs, after all) as evidence of what weight savings one-shot racks
could allow if somebody -- heck, anybody at all -- in-universe decided the idea was worth enough to finally stop half-assing their attempts at implementing it. (Even the "improved" versions quickly get ridiculously heavy, after all, which tells me right there there's plenty of potential left.)
But as long as that doesn't happen, I don't really see any more effort being put into creating specifically weight-efficient one-shot Thunderbolt launchers either, so as far as I'm concerned they'd likely end up simply being more of the same.