Each election in India has its own features and backgrounds. An important factor is the political situation in the states, the setting of coalitions and agreeing on candidates for constituencies (in a majority electoral system coalition partners don't want to take away each votes) as well as the perceived performance of the Government. In a multicultural society such as India also regional, ethnic and caste constellations (reservation) play an important role, and the stand of rural and urban issues (Bharat vs. India).Until the mid-1960s the dominance of Congress was almost everywhere in the state and the federal government.

At federal level it took until 1977 before Congress experienced its first loss of an election and Janata Party under Desai and later Singh came to power, mainly as a reflection of the undemocratic rule and state of emergency by Indira Gandhi before that. However the Janata Party wasn't stable at all and in 1980 Congress came back and in 1984 it not only consolidated its power, but achieved its ever best result. The sympathy vote after the election of Indira Gandhi was crucial (later in 1991 her son Rajiv was assassinated in the middle of elections. In Tamil Nadu where the assassination happened it split the election into constituencies where elections were held before the assassination and such where voting was done after. The results for the latter was much, much higher for Congress than the results for the constituencies before the assassination. By the way: the 1991 election was preceded by a non-Congress government (V.P. Singh and then Chandra Shekhar), which again showed to be far to heterogeneous to e viable with parties from the far right, central and left (communist parties were outside, but backed the government....)

The elections of 1984 brought Congress almost 50 percent of the votes and more than 400 seats. 2014 the BJP reached 282 votes and some 31 percent of the votes. Here is already a fundamental difference. The BJP was supporting the  V.P. Singh's government of 1989 from outside, but played an important role by highlighting Hindu nationalist sentiments (e.g. Ayodhya dispute and Ram Rath Yadra). Then the VIPs of the BJP were Atal Bihari Vajpayee and Lal Krishna Advani. Singh resigned as the BJP withdraw support after thousands followers and also Advani were arrested in connection to the Ram Rath Yadra. Chandra Sekhar, with support of the Congress from outside took over with less than 70 MPs in his government. Like earlier the BJP this time the Congress did not want to dirty their hands being in the Government. This then helped Congress a lot in 1991, but the major issue was the assassination of Rajiv Gandhi.

Rao (Congress), Vajpayee (BJP),  Deve Gowda, Gujral (both Janata Dal) followed with little political stability. Every election saw part of the opposition winning, before first Vajpayee (BJP, 1998 for six years) and then Manmohan Singh (Congress, 2004 for 10 years took over). These two were the only PM since 1984 that were re-elected.

It becomes clear that today political leadership at national level requires coalitions. Last time in 1984 Congress theoretically could govern based on its power in Parliament. With the need of coalition governments political fragmentation is greater. Also regional parties such as DMK and AIADMK in Tamil Nadu or the Telugu Desam in Andhra become more powerful, being able to blackmail a national government to withdraw support.....

Making it short: to analyse election results in India requires much more than comparing seats won or lost. One need to look at the set up (like I wrote: when coations arrangements are there then parties agree not to contest in particular constituencies....). One lao need to see, how parties are able to bring a rather heterogeneous electorate behind them.