Oil Futures expire every month. When expiry occurs one must sell out of the expiring contract (if long) and buy the next month's contract to maintain exposure. This is called 'rolling' futures forward and is how the index (SPGSCLP) will achieve it's return. However, when it 'rolls' a roll yield is incurred. This is negative when the oil futures curve is in contago (upward sloping). This is the case at present. So when the futures rolled in March (say oil was at $48), you sell this expiring contract and buy the April contract. Unfortunately you cannot buy this new contract at $48 and have to pay say $50.4. You have incurred a 5% negative roll yield. Same occured in April and hence the 10% difference. Sorry if I went overboard with the detail. Hope this helps.