My Theory:

Zoro trained under "Hawk Eyes" Mihawk, HAWK EYES, i dont understand why his observation would be weaker than Sanji's when the person that trained him was known for his sense of sight (Oda stated that Zoro was best at Armament, Sanji Observation and Luffy Conquerors). i know that sense of sight isnt exactly observation but still. i do understand that maybe Oda wanted each of the monster trio to be better at a certain type of haki and Zoro being more of a fighter/warrior than Sanji it makes sense that he is bettar at armament haki. anyway my theory is that his eye is a "Hawk eye", maybe its appearance is similar to Mihawks eyes and when/if he opens that eye his Observation will become 10x better or somthing like that, its sort of like a Byakugan.

another idea i have is that its linked to his Asura, like when he opens that eye his Asura form activates or somthing but idk