Really good question, it is very confusing with SBD no longer being pegged to USD.

1.35 SBD + 0.77 STEEM is what the payout would be from that particular vote after curation. This is in fact a 50/50 split since 0.77 STEEM at the current price of $1.75 is equal to $1.35.

So the 50/50 split means you get half of the payout value in SBD (which steemit.com assumes to be equal to USD) and half the payout value in STEEM.

The $15.56 amount comes from multiplying 1.35 SBD by its current price in USD which is about $10.50 + 0.77 STEEM * its price which is 1.75. So:

1.35 * 10.50 + 0.77 * 1.75 roughly equals $15.56 (I rounded some of the values for simplicity)

Hope that is clear!