First of all, it's important to realize that the speed constraint at MC768 is not 230, it's 230 or less.
If it's targeting 220, it's because it thinks it needs to be at 220 now in order to make the speed restriction further down the line. I don't understand why it thinks that and as suggested, pulling speed and setting 230 until a few miles before 768 may work out fine but if it doesn't then it will be obvious what is going on.
Are there any previous restrictions behind the aircraft? Maybe it's on a geometric descent rather than idle and this is causing the calculated 220.