I asked IB this question and they are now treating ARCA in a similar fashion as AMEX and NYSE (in terms of odd lots). You can't initiate odd lot positions on ARCA (a 99 share order is entirely rejected, a 199 share order would only be filled for 100 shares, the other 99 would be ignored). You can close out odd lot positions on ARCA. ARCA has odd lot abuse guidelines that are similar to the AMEX and NYSE so that may be one reason IB has chosen this routing method. Another reason is the cost of listed ARCA odd lot orders is much higher than other order types.