SPY Box Spread - Negative Yield??

I am pricing box spreads and noticed that the SPY box is implying a negative yield (i.e. to buy the box you pay more than the width of the strikes).

Meanwhile, the corresponding SPX box is trading at an annualized ~5% yield.

I selected the May expiration cycle, so in theory there is no ex-dividend risk. Why would anyone pay more for a box than the width of the strikes? What am I missing here?

Thanks,
Bobby

 

SPX is 10x SPY,

so 1021 is not that much different than 994 when you account for the spread differences.

 

I understand the sizing differences between these products. And spreads on these products are super tight, so I don't believe it's spread differences...

It's more that the underlying financing markets are materially different compared to normal (the annualized yield in box spreads between these products usually trade around the same rates). My only thought is that it could be ex-dividend risk, but there's no dividend between now and the May 16th expiration...

 

I think it has to do with American exercise ability.

 

Early exercise risk? is it not true that you should not do a BOX spread on SPY but on SPX