1dollar bill equals $1.
4 quarters equal $1.
10 dimes equal $1.
20 nickels equal $1.
100 pennies equal $1.
So sometimes you have different coin worth different amounts and depending on how many of each coin you have they can add up to $1.
BTSX and bitUSD are similar in this same way.
BTSX are the coins that can add up to 1 bitUSD. The only difference is that people get to decide how much value BTSX has every day, hour, minute and second relative to a dollar. Thus, BTSX will constantly be changing from a "Penny" coin one day, to a "quarter" coin on some other day.
2 days ago, 115 BTSX equaled 1 bitUSD (approximations)
Yesterday, 33 BTSX equaled 1 bitUSD
Today, 40 BTSX equals equals 1 bitUSD
Here is the kicker: 1 bitUSD always equals $1.