Meanwhile, the iShares iBoxx $ High Yield Corporate Bond ETF (NYSEArca: HYG), the main competitor of JNK, has returned a slightly better average annualized 4.80% over the past five years, compared to an average 5.46% return for the benchmark Markit iBoxx Liquid High Yield TR USD Index.

The slightly better or less worse tracking error found in HYG may be a contributing factor to its growing success. HYG has seen assets surge by 50% since August 2014, compared with the 25% growth in JNK’s assets under management.

Related: Don’t Overlook Junk Bond ETFs to Help Meet Income Needs

Industry experts attribute tracking error to how ETFs are constructed, or “optimizing.” ETFs employ a type of sampling technique that only picks certain securities from an index. Fully replicating an index is not always practical, especially in less liquid markets and hard to access areas, such as the notoriously illiquid speculative-grade debt market.

For more information on the fixed-income space, visit our bond ETFs category.

UPDATE: added additional information from State Street Global Advisors.