Because lance terminology is dictated by mass, so this is an arguably false supposition? A quartet of Whitworths is a medium lance, but a quartet of Trebuchets is a heavy lance, even though the 'Mechs are used in the same role and have similar weapon loads. Hell, the Trebuchets are faster, so you think they'd get called the medium lance instead, no?
Because you don't always have homogenous lances. Look at the Black Widow Company (circa 3025). Command lance is a Warhammer, Marauder, Crusader, and Griffin. 265 tons. And it is most assuredly a heavy lance. Fire Lance has two Archers, a Stinger, and a Wasp. 180 tons. Medium Lance. Recon Lance with Rifleman, Phoenix Hawk, and two Stingers. 145 tons, and it is a Medium Lance again.
As to why the lances don't represent actual 'Mech weight classes exactly? It doesn't have to. Four Jenners, or four Firestarters, or four Panthers is effectively a Medium Lance, although on the low end, just like two Centurions, a Treb, and a Hunchback are the high end. Same for two Griffins, a Shadow Hawk, and a Wolverine, except that is a low-end Heavy Lance, whereas two Thunderbolts, a Grasshooper, and a Warhammer make up a high-end Heavy Lance. And God forbid you should ever run into a lance of four Orions. That
is an Assault Lance, my friends. Even though there isn't a single rare assault-weight 'Mech in it.
It is just how it has always been.
Master Arminas