Pourquoi est-ce que c'est « Bien qu'elle se soit » ?

Vinit, 

Took me a while to get my head around this too.  

Apart from ' bien que ', this is not a fixed expression.

The verb is the reflexive ' se spécialiser ' in the past subjunctive feminine singular form - se soit spécialisée - and 'elle' is the subsequently mentioned ' la société ',  qui  ' se diversifiera '. 

Hence, paraphrased for clarity, ' bien qu'(elle se soit spécialisée) à l'origine dans .... '

(The relevant noun here appeared 2 screens later, but until then, assuming the reference was to the aforementioned ' l'entreprise ' results of course in the same construct and agreements, as ‘Elle’ is the given subject pronoun)

 https://www.wordreference.com/conj/frverbs.aspx?v=se%20sp%C3%A9cialiser

 https://www.wordreference.com/conj/frverbs.aspx?v=se%20diversifier