Valve are terrible mappers
I found that the quality of mapping varies from test chamber to test chamber. Some chambers are incredibly detailed, while others have misaligned textures and are sloppily put together.
I just played through the entire game again from a mapper's perspective. I noticed that some of the indicator lights do not even have those "corner" embellishment. There is no consistency in when those corner lights are used either--sometimes it is there at the "target" entity, and sometimes it is not.
I can tell you that Valve has teams of level designers that are responsible for individual chambers. The variation in mapping quality is very apparent.
Vengeance
Olympic
[SP] Interception
Interception PeTI
Herman
Uprise
Unified
Well as a player you are more likely to care about the ambience or the gameplay, rather than consistency of a small object. If Valve had OCD tendencies when it came to consistency, none of their games would ever be finished. I mean... we wait long enough for their games already... sheesh!
I'm very sure that's why they introduced instances. They do have a large team to put together many maps in a particular timeframe, and they need it to look similar throughout the game.
Instances are a great time-saver. It's the more evolved version of prefabs.
Vengeance
Olympic
[SP] Interception
Interception PeTI
Herman
Uprise
Unified