Intel has never provided their reasoning behind this decision. If you want to read some of the (often silly) industry speculation on the subject, you can start here: https://www.quora.com/Why-did-Intel-name-their-processors-as-i3-i5-and-i7-instead-of-i2-i4-and-i6-or-i1-i2-i3-i4-and-so-on.
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