In this option chain for SPY Apr 25 calls
the one with a strike of 170.50 has a lower price than the one with a strike of 171. But why is this? 170.50 call is deeper ITM than 171, so it should be more expensive, right?
You are looking at last trade. The current bid/ask is more relevant. This issue is addressed in your recent question on SPY options as it relates to volume and current validity of posted prices.