IEX 'deep' says avg spread for DJIA is great than that for all stocks?

In [4]: %run spreadratio.py
spread ratios
all symbols: 0.029280614424745024
IB ETF symbols: 0.005647272013423552
DJIA symbols: 0.03525528539759046
SnP500 symbols: 0.034131710271758486

I would have expected almost the opposite (except for IB ETFs*) because the average DJIA symbol is more heavily traded than the average SnP500 symbol which is more heavily traded than the average symbol.

Wouldn't the market makers be all over the more heavily traded symbols?

*ETFs came out the way I expected more or less since they're composite symbols.

 

Are these spreads price adjusted?

 

spreadratio[sym]=(ask-bid)/((ask+bid)/2)
WHERE
ask is the lowest ask
bid is the highest bid

 

I suppose IEX Deep shows you the best IEX bid and ask. Maybe IEX is more prominent in lower cap stocks and not very often top of book on the large caps? I agree your stats are unexpected.