I'm looking at options chains for SPY for some particular expiry date.
I'd expect that the last trade prices would vary smoothly with the strike price.
But they don't; there are occasional puts that seems especially cheap. That's ok; if I'm not particular about the strike price, then I look for these and buy them.
Is there some particular reason for this unevenness, other than perhaps lack of liquidity? Note that SPY is apparently the most widely traded option.
Why don't arbitragers step in?