Lal Bahadur Shastri’s actual surname was Srivastava, Shastri was the title he got from Kashi Vidyapith when he graduated with a first-class degree in philosophy and ethicswhich he used. Apparently like most of the freedom fighters those days, he did not believe in caste, so never used his original surname.
Same with Ishwar Chandra Vidyasagar, whose real surname was Bandopadhyaya. He got the title of Vidyasagar from Sanskrit College, Kolkata, where he graduated from, due to his excellent performance in Sanskrit studies and philosophy. And he used that along with his name.
Chandrashekhar Sitaram Tiwary, took the moniker of Azad, apparently after a well known incident, when as a 15 year old teen, he was arrested for protesting against the British rule. When the jailor asked him his name, he shot back defiantly “Azad”, and that became his more popular name.
Actually that seemed to be common with most of the revolutionaries of that time. Ram Prasad used one of his pen names Bismil, along with his first name, by which he became more popular.