How does sizing of ar15 handguards work?

My current setup is like this. https://imgur.com/gallery/UASgZLi I just want to replace the handguard with a new one. I was searching for 12.5” handguards because that’s closest to mine but mine is actually 12.65” and the distance to the gas block from the receiver is 12.8”. I can’t choose a 13” handguard because... show more My current setup is like this. https://imgur.com/gallery/UASgZLi
I just want to replace the handguard with a new one. I was searching for 12.5” handguards because that’s closest to mine but mine is actually 12.65” and the distance to the gas block from the receiver is 12.8”. I can’t choose a 13” handguard because it will conflict with my gas block but I am worried a 12.5” will excessively reveal my gas tube and leave it vulnerable to damage. I’m confused as to why my handguard is 12.6” as well because that isn’t an option for purchasing anyways. Is it a rounding thing? That seems weird because people would plan out their dimensions based on they’re tubing potentially and extra length could compromise their plans.
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