If you're talking about the four G5 additions to the big 12, they took a reduced share because ESPN refused to pay for them. It lasts only until the next contract begins, when they will receive a full share. It isn't based on some type of generalized rule that says all new members must accept a reduced share for two years. There are specific facts and circumstances for each situation.
Keep in mind that ESPN and Fox have agreed to pay the big 12 a full share, not a partial share, for Connecticut. Accordingly, the same justification that was used to pay the last four additions a reduced share is not applicable to the addition of Connecticut, just as it is not applicable to the addition of Colorado.