You can check a previous statement to see if the percentage in Bond Market R1 was higher or lower.
The reason is that the funds change in value at unequal amounts.
For simplicity sake, imagine you originally had $10,000 and selected two funds at a 50/50 allocation. Fund 1 grows 10% a year, and Fund 2 grows 5%. After one year, you will no longer have a 50/50 allocation. Fund 1 has $11,000 and Fund 2 has $10,500 for a total balance of $21,500. TIAA-CREF would rebelance so that $21,500 is once again evenly distributed.