Why is iso-octane different from other iso-s?

While learning nomenclature of organic compounds I found the common names to be a bit difficult to learn than the IUPAC ones. One of these common name stuff is the iso-,neo-, sec-, etc. So I searched for the ways to differentiate between these on the YouTube and found [this video](https://youtu.be/UPr8Z9SRnL0) (note that the anchor of the video uses hindi for communication so it might not be intelligible to most people here). To summarize what the anchor said:



> 
> **Prefix-iso**: is used when all carbon atoms **except one** form a continuous chain.
> 
> 
> $$\mathcal {OR}$$
> 
> 
> They have a structure like this:
> ![General Structure for iso- compounds and an example](https://i.stack.imgur.com/2aZnMl.jpg)
> 
> 
> 


Now in the same video the anchor says that iso-octane is an exception to this rule, as it has the following shape:
![Iso-octane structure](https://i.stack.imgur.com/rKw15.png)


Before this came in the video I formed the view that these are just the ways of naming compounds with similar structure but now I think that that isn't the case. So:


* Why iso-octane has shape different than the other iso-s?


The anchor says that the structure of iso-octane was determined experimentally. This implies that there is some way to identify these compounds without knowing their structures beforehand so:


* How do we know that whether some given compound is iso-, neo-, etc?
* What might be the most general guidance for using iso-, neo-, sec- and tert- prefixes?




The usage of non-standard prefixes such as "iso" has never been defined with hard, unambiguous rules such as those in the IUPAC recommendations. Instead, their usage has often more been "by convention". In *Organic Chemistry* (1st ed.) by Clayden et al., the authors write (p 315):



> 
> Strictly speaking, if the standard meaning of ‘iso’ is followed, the name isooctane should be reserved for the isomer 2-methylheptane. However, 2,2,4-trimethylpentane is by far the most important isomer of octane and so, historically, it has been assigned this name.
> 
> 
> 


(This phrase is also found at [Wikipedia](https://en.wikipedia.org/wiki/2,2,4-Trimethylpentane), where it is copied wholesale without any indication that it's a direct quote. Please don't do this in your work. Even if you provide a citation, you should either paraphrase the information, or clearly indicate that it's a quote, e.g. by enclosing it in quotation marks.)


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