marginal relief

its purpose: increase in tax should not be more than increase in income. comes into picture when your income is just above Surcharge threshold.
it's easier to understand absence of marginal relief. if it was not there, then a person with income of 1cr and 10Rs will have to pay 4.23 lakh more tax than a person with income of 1Cr.
If you want the equations behind these do let me know, because the case becomes even more interesting for FY 2017-18 as there are two thresholds.