Reraise 3 way with allin included, confussion

Hand went like this ($0.10 / 0.25$ blinds):

CO raises to $0.85

BU calls

SB calls

BB raises to $4.25

CO calls 

BU folds 

SB raises to $8.74

BB raises to $17.08 

CO calls 

SB goes allín 

My confussion is then how SB went allin if BB raised to $17.08 which isn't the double of $8.74. 

Thanks in advanced!