Prime Ministers expand or reshuffle their Cabinet for a variety of reasons. Their decisions have little to do with merit and almost everything is to do with politics.
Rajiv Gandhi had a brute majority and used it to toy with his ministers. He rewarded loyalists and punished those out of favour with painful regularity, setting a record of 23 reshuffles in just 38 months.
Atal Behari Vajpayee was forever under pressure to balance the demands of his party and his coalition partners. He was not as whimsical as Gandhi but still ended up shuffling his pack eight times in the five years he was in office. The last time was a few months before the general election and earned him brickbats for using the tool of Cabinet expansion to re-induct just two people. Both were allies who had proved to be untrustworthy but who had to be accommodated for electoral reasons.