SCOP has slight bias towards even numbers (first pointed by Paul Crowley here). With probability 257/512 == 0.501953 != 0.5 in the generated key stream is produced even 32-bit word, which means that with probability 0.501953 != 0.5 every least significant bit of every cipher text 32-bit word will remains the same as the corresponding plain text bit. Note, that this bias concern only the lowest bit of the produced 32-bit keystream words. SCOP has this even number bias on one hand, but on the other hand SCOP reach an excellent speed - least than 2 clocks per encrypted byte on Intel Pentium processors, and also SCOP is free.