We'll your point about delta's is likely correct in general and on average. However, SPY contracts are all independent and traded in significantly different amounts. For instance, the 209 contract sees much less volume than the 208 and 210 points just because people tend to prefer round numbers.
Many market makers may continuously update their 208 and 210 numbers showing the full range of the day while their 209 mark may update infrequently or only when someone asks for a price. So the range of the 209 may not reflect the full range of the day.
Also there are other things that affect price variation throughout the day rather than just the underlying price. So this is another possible reason. Probably less likely though as 208 is so close to 209.