Eg, suppose FOO is trading at $100/share, and you have $5k of cash and $5k of free margin. You use all $10k to buy 100 shares, then sell a call option with a strike of $60 for $40/share.
Under RH's calculations, you have $5k of account equity, $10k of stock and $4000 of cash from selling the call. This qualifies you for a margin loan of up to $9k.
Under the correct calculation, the stock is only worth at maximum the $60/share strike price of the short option, since that is the cash you'd get if the option is exercised. So you have $5k of account equity, $4k of cash, and $6k worth of net marginable securities.
The advantage of this is that it treats out-of-the-money covered calls better. If you sold a call option on FOO with a strike of $105 for $2/share, you should have $200 extra free to spend on stuff. Selling extrinsic value should wind up generating net cash that users can use for whatever purpose they want. Selling intrinsic value, on the other hand, is selling a portion of the economic right to the underlying.