Light sources that are generated by weapon attachments have a different perceived brightness in multiplayer situations. The player whose weapon is hosting the light (first player) sees the light precisely as it is displayed in singleplayer situations. Players who are not the host of the light (third players, for convention's sake) will see the light change brightness according to certain conditions.
Assume conditions to be a cloudless night with a new moon, but the specific darkness of the night does not seem to have an impact.
When the weapon light is turned on, to a third player it appears very brightly, at first. After about a second the light will dim down (over about another second), so that both first and third players will experience the same brightness from the light.
If the first player is pointing their weapon at a nearby object, the light has adjusted itself as described above. If the first player is not pointing their weapon at a nearby object, but rather a distant one (such as a hillside or the sky), the adjustment will not occur.
If the first player is pointing their weapon at a nearby object, and the light has adjusted as described above, and then points their weapon at a distant object (such as a hillside or the sky), the light as perceived by a third player will become brighter until it is as bright as when it was first turned on. This can be readily seen if there is still a nearby object within the cone of the light. Returning the weapon to pointing at a nearby object will make the light return to an appropriate brightness.
Finally, if the first player points their weapon at a nearby object and doesn't move, then the light, as seen by a third player, will "pulse" slightly brighter every 3 seconds, before returning back to being dim.
These effects can be seen demonstrated in the following to videos:
https://youtu.be/WXn47ELduMs (58 seconds, demonstrates changes in brightness for an intensity value (effectively) of 200)
https://youtu.be/aHdndcOenf8 (67 seconds, demonstrates changes in brightness for an intensity value of 90)