Imagine author rewards are $2 (curation has been dispensed), and further imagine STEEM = 40 cents and SBD = 80 cents.

At 100% SP you get 2/0.4=5 SP (STEEM). At 50-50 you get 1/0.4=2.5 SP plus $1 is turned into 1 SBD worth 80 cents = 2 STEEM if sold on the market.

2+2.5 < 5

The inequality holds whatever the price of STEEM, so long as SBD<$1.