i think you need to take into account the opening price too....

i think you need to take into account the opening price too. BBUS gets repriced on open based on the overnight movement.
that opening price and S&P500 point in time is the start point.
the movement intraday in BBUS is all relative to that start point. Then at close BBUS is repriced again against the start point.

today is a great example.
futures droped 5% or so over night. BBUS opened strongly. From that start point futures are up 1.5% ish. Depending on how BBUS trades today it may get repriced on close higher or lower than the 4pm market price based on futures reletive to the start point

(i think)