I'm out of practice, but I believe it's an irregular plural - much like "goose/geese" or "mouse/mice" in English. Some words, particularly well-used ones, will not follow the pattern that most words follow. This should be better explained on the tool tip at the introduction of this word.
It is perfectly okay to ask why and every seemingly illogical and irregular feature in languages has a logical reason and interesting history behind it, and it opens up a whole new world of knowledge to one if one just bothers to explore more. Back to the topic: Italian word uomo derives from Latin word homō, which also means man, mankind. Homō is a third declension word and it is in nominative case, its root is homin-, and its nominative plural is hominēs, which developed into Italian plural for uomo, uomini. I recommend Wiktionary, it can sometimes provide quite detailed etymological information. http://en.wiktionary.org/wiki/uomini
uomo comes from Latin homo, pl. homines, a third declension noun (declension = noun conjugation class) but the 3rd declension pl. -es ending was replaced in Italian by the originally second declension pl. ending -i, as in canus (dog), cani (dogs). Basically Latin had 5 declensions, and as it developed into Italian, 2-5 were all merged into 2, becoming the masculine, and 1 became feminine (so for 'girl', the Latin was puella sg., puellae pl., as in Italian ragazza, ragazze).
Basically, the extra bit in the plural is a relic from the Latin root.
(P.S. I suspect actually the fate of the Latin 5th and 3rd declensions is probably a bit more complicated than I've described here, but I'm no Italian expert(!), and it serves to demonstrate the point here at least.)
"siamo" means "we are"; "sei" means '"you are". We use the same verb in English and therefore need the we/you pronoun; the pronoun is optional in Italian because the verb already carries the we/you meaning. So siamo and sei both translate to 'are', but one is for 'we', and one is for 'you'.
I'm very confused because in some sentences for example "they are women" the traduction is Sono Donne but in another one it says "They are boys" and the traduction is Loro sono ragazzi so my question is Why sometimes the pronom doesn't appear even when the meaning of both sentences say the same?
Subject pronouns are optional because the conjugation of the verb indicates who does what. "Loro sono donne" and "sono donne" mean the same thing. This would be pretty confusing in English because "(we) are women" and "(you) are women" would look the same... but in Italian we and you have different conjugations.