For options shorting exposure margin is calculated as below

"The exposure margins for stock options

- For option contracts on individual Securities:

The higher of 5% or 1.5 standard deviation of the notional value of gross open position in futures on individual securities and gross short open positions in options on individual securities in a particular underlying. The standard deviation of daily logarithmic returns of prices in the underlying stock in the cash market in the last six months is computed on a rolling and monthly basis at the end of each month.

For this purpose notional value means:

- For an options contract - the value of an equivalent number of shares as conveyed by the options contract, in the underlying market, based on the last available closing price."

Copied from NSE site, can check that for more info on margins.

That way of calculation gives exposure margin of around 30%, also as yesbank is under ban due to MWPL limit it has higher exposure based on the slab of MWPL it fall.

Can download full circular from here, no 31685.

Hence it is charging much higher exposure, you logic is theoretically correct but practically exposure is charged based on various other factors, hope this answered your query.