The vast majority of major bond indexes are calculated based upon trader prices. As apposed to market value. What does this actually mean?
Bonds do not trade like stocks. There is not really a consensus on the price of a bond at a particular point in time. Some may say +100/10yr another may say +99/10yr. Not to mention you will get a different price buying 10,000$ in par vs. 1MM in par. So, the market can’t price, it has to be a trader, or or traders for a specific index. That is why there are bond pricing services for portfolio managers to benchmark the value of their holdings.