There is a chance the weapon you are trying to dismantle for infusion has a legendary mod on it, something like kinetic damage or a damage type. All legendary mods add a plus 5 light level to your item. So if you've got a 270 and you have a 275 you want to infuse into it, there is a chance that that 275 is actually a 270 with a legendary mod. You can tell by opening the weapon and looking it the mod slot to see if the mod is legendary or not.