* Old English roots: The numbers 11 and 12 ("eleven" and "twelve") actually have roots in Old English that are distinct from the regular counting system. "Eleven" comes from "an + lif", meaning "one left over" (after ten), and "twelve" comes from "twelf", which was a separate word.
* Teens are different: The "teen" numbers (thirteen to nineteen) also have a unique structure, but they are based on the number words for three to nine with "-teen" added on.
* Regularity: Numbers from 20 onwards are formed more regularly, using the base numbers (twenty, thirty, forty, etc.) and then adding the singular number words (twenty-one, twenty-two, etc.).
So, while the logic of "onety-one" makes sense based on the later patterns, the words for 11 and 12 are historical holdovers with their own unique origins.