With all due respect, I do not think you can make such a conclusion from a 1.6% difference over a one month period, or even a longer period for that matter. The reason is that on-time arrival performance into a particular airport is influenced by the point of origin for each particular flight. Just because an AS flight arrives late into SEA does not mean the delay is either caused by AS operations or SEA airport conditions. The delay could have been caused by weather in JNU or KTN or SIT or some other city they served. Just like an AA flight delayed into SEA could have been delayed due to thunderstorms in DFW. Also, note that many weather delays are in fact categorized under the "national aviation system" delay category in the Air Travel Consumer Report, so one cannot simply look up the data for JNU and say, oh there were no weather delays since it says 0 flights delayed due to extreme weather. If the weather is "extreme," in most cases the flight will divert or just be out-right-canceled, and they do not list the causes of diversions or cancelations.