Prefixing `一` to numbers

It seems to me that 千 is exceptional in having it prefixed with 一.

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  As for 一, 十, 百, they work together with 千 to describe the numbers from 1 to 9999, so they compete with one another, or are complimentary. There is no combination like:

  
  
  

  
  

  一一, 一十, 一百, 十十, 十百 百百

  
  

  
  


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  As for 万, 億, 兆, 京, etc., they are multiplied by a number from 1 to 9999, expressed by the prefix using 一 十, 百 千, so when that prefix turns out to be 1, 10, 100, 1000, then they are combined, and there are combinations like:

  
  

  
  

  一万, 十万, 百万, 千万, 一億,　十億, 百億, 千億, etc.

  
  

  
  



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  and there is no combination among themselves such as

  
  

  
  

  万万, 万億, 億億, 万兆, 億兆, 兆兆, etc.

  
  

  
  

However, 千 is exceptional in that it is prefixed by 一 just when itself is prefixed to the characters 万, 億, 兆, 京, etc.

千

一千万
一千億

Why is 千 prefixed with 一 in this case? Why is it not like the following?

千
千万
千億

Answer

I'm mostly guessing here, but I think it might be in order not to mistake them for 三万 and 三億 when spoken. While せんまん and さんまん only differ by a vowel, いっせんまん and さんまん are easy to distinguish.