The S&P 500 has endured a tumultuous year that bottomed with the late-summer sell-off but has rallied of late back toward its record-highs. ETFs that track the index have of course risen back with the index, but depending on the methodology they use to track it, results are varying widely – and that’s a bit concerning.

Exchange traded funds are most commonly passively managed, meaning they track an index using a certain set of rules and weightings. The S&P 500 is one of the most commonly tracked indexes in the world and the SPDR S&P 500 ETF (NYSEarca: SPY) uses the sames weightings as the S&P itself. However, when you compare SPY to the Guggenheim Equal S&P Weight ETF (NYSEarca: RSP) that tracks the same index, but weights its holdings equally instead of by market capitalization, you see a divergence.

Looking at Spy (blue line) & RSP (red line) movement over the last month shows that most of the time their spread is negligible but has widened over the past few weeks. Why is that? Well, since SPY is market-cap weighted its enjoying the advantage of top-heavy performance to carry it higher than its equal-weighted cousin.