Its probably a pulsed laser.
The Bolter is not made to deal with heavy armour, but utterly devastates most things in guard flak or light carapace. I think part of the deal is that there's some thresholds where a weapon actually is effective, as opposed to one just a wee bit weaker.
If we look at three different targets, with minimum, average[we'll say 6 for the lasgun and 7 for the boltgun] and maximum damage off an overloaded lasgun [1d10+5E AP2] and boltgun [1d10+5X AP4]. This gives us values of 6/11/15 AP2, vs 6/12/15 AP4 respectively:
Man in AP4 Flak with Tb2 with 8 wounds
Lasgun: 2/hit minimum, 7/hit average, 11/hit maximum.
Boltgun: 4/hit minimum, 10/hit average, 13/hit maximum.
Against normal folks or cultists, both will average two hits for a kill against the target, though the boltgun generally puts a low-wound target into the early criticals with a single strike.
Trooper in AP 6Carapace with Tb3 and 11 wounds.
Lasgun: 0 minimum, 4 average, 8 maximum
Boltgun: 1 minimum, 7 average, 10 maximum
Here, you'll probably need a fourth lasgun hit of average damage cause significant critical damage. The Boltgun on the other hand toppled the guy in two hits, and is unlikely not to have killed him with the third.
Rogue Trader with AP 9 and Tb4. 16 wounds.
Lasgun: 0 minimum, 0 average, 4 maximum
Boltgun: 0 minimum, 3 average, 6 maximum
Don't bother with the Lasgun unless there really is nowhere to run. Meanwhile, the bolt-weapon still works okay. its lowest rolls can be shrugged off, but on average you still actually stand a chance. At this point, Special Ammunition would probably improve your odds significantly, something a lasgun cannot use.
I'm not stating this is the real reason for how things are set, but I'm thinking the values chosen for output are as they are because one weapon can kill what the other cannot scratch, and with a modicum of efficiency at it. That lasgun was really getting pushed there after all.