Yeah, it's because the alert code checks the classname to decide what alert sound to play, and doesn't have an extra check for monster_ogre_marksman. Other places this matters are infighting - monster_ogres will attack monster_ogre_marksman, and obituaries, which are uncredited if the marksman gets the killing blow. Most interestingly, while the monster_ogre uses OgreCheckAttack, the marksman uses the generic CheckAttck function to decide when and how to attack.
The differences between the two are that OgreCheckAttack always melee attacks if at melee range, and always fires grenades if enough time has passed since the last attack and line of sight is not blocked. CheckAttack is more probabilistic, with 20% chance per frame of a missile attack if near, and 5% per frame if mid range. On the upside for the marksman, the minimum time between attacks is shorter, randomly between 0 and 2 instead of 1 and 3 for the ogre. So at short range the expectation is the marksman fires faster, at mid range the regular ogre comes out on top, on average. Pretty much all of the marksman's advantage is gone in nightmare.
If you look at the OgreCheckAttack code you might see "chance" assigned various values depending on how far the ogre is from the target. This is all totally redundant, as the function then goes ahead and fires anyway without testing chance. Seems most likely this is leftover code from the generic function that should have been deleted, not an intended design that should have been fixed up.
One other thing I thought of about the AI here is that it shows you how the chance function is not a good way to try and make the enemy close the distance, firing occasionally. Even with a low probability like 0.05, we expect a shot every two seconds. If you're designing your own monster, and you want them to occasionally lay off the attacking to get close, a more reliable way of doing it is to set an attack_finished time.
A crude decision process would be to randomly decide with p = 0.7 say to not attack for a while and instead close in on the player. Then set attack_finished to time + 2, and the monster will just run at the player for that wrong, then decide if they need to melee or close in further, or if this time they should attack. It's useful because it allows you to make a command this frame that affects the ai for several frames in the future. Making better decision than random number generators would be the next step, like deciding to close in if the player has a RL out.