The plural of braccio is braccia, and the plural of avambraccio is avambracci.
Why are the plural of those words so different, if they both are referring to parts of the human body, and avambraccio derives from braccio?
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As far as I know, some words have two plurals, and there's no general rule about them. In the case of braccio, it can be related to human body, or to something else (e.g. i bracci del lampadario).
The same applies to a lot of other words, such as grido, filo, corno, etc.
There is no other meaning for avambracci, so there's no need for a different plural form.
So, bracci actually exists as plural, but I believe that a slightly different version (braccia) found its space in language to mark in a stronger way the different meaning.
Anyway, I don't know the exact reason why speaking about the human body braccia has been chosen instead of bracci. Also, a legit question would be why it applies to plural but not to the singular word, which is the same for different meanings.